Yet another brake question

Sorry this is going to be a long one...Last fall I finished building my new to me RX 7 track car, most of the components were moved over from my old car and as such expected the new car to be very similar to the old car. The new car had somewhat sketchy brake lines so I replaced them all. The pads and rotors from the old cars were still good so I kept them. In preparation for its first event in October I bled the brakes numerous times. When bleeding them the pedal was nice and firm, about an inch of travel, all seemed well. When I got to the track the brakes stopped the car fine but the pedal traveled about 2" or so before the brakes engage. In doing some trouble shooting it looked like I might have a problem with the check valve in the vacuum hose to the booster. It turns out the old one was blocked and I installed a new one over the winter. So the other day I installed new pads and rotors and did three passes around the car bleeding the lines. No indication of any air in the lines, nice firm pedal with the engine off. Today I took the car out for a quick ride around the neighborhood to bed the brakes and once again the pedal is travelling way more than it does with the engine off. Once the pedal engages the pads they stop consistently and the pedal doesn't sink. What am I missing, the old car had about an 1" of travel whether the car was running or not.

In reply to imgon: Two thoughts.

1. Is the pushrod adjustable? It's been 16 years since I had an FC and I don't remember anymore if they do or not. If it is, the pushrod may be adjusted shorter than the old car.

2. Possible bad master cylinder with the first circuit leaking internally and producing all the braking force with the secondary circuit. The brake booster produces enough extra force that what feels solid with no assist can be really squishy with the assist working.

If you've got the 4 piston front calipers, you should have plenty of braking capability if everything is good.

First off, of course the pedal travels further with vacuum applied to the booster. Thats kinda the point of it.

If your old car had the same travel off and running, the power brake booster wasn't working.

In reply to Streetwiseguy: I didn't realize the booster would effect the pedal travel. I thought it just gave an assist to the pressure applied. If the pushrod is adjustable, should I be able to get the pedal back to only travelling an inch instead the two or so it does now? I'll have to dig out the FSM and see if it is adjustable.

Some quick checks for you to do.

With the engine off, press the brake pedal several times to get rid of any residual vacuum boost. Once you've done that, press down firmly on the pedal and hold it for 20-30 seconds. The Pedal shouldn't creep. If the pedal creeps, you're looking for a leak somewhere. If not, then onto test 2

Next, provided you've got no leaks, depress and hold the brake pedal. Now, while keeping the pedal pressed down, start the engine. Once the engine starts, you should feel the pedal sink a bit. That's an indicator that your booster is functioning correctly. The pedal should still feel firm after sinking and shouldn't go to the floor.

After that, take your foot off the brake pedal and give the car a couple of revs. Shut it off and wait a minute or two then press the brake pedal a few times of times. You should feel the pedal travel shorten up back to where it was at test 1. If the pedal doesn't retain the boosted feel for a couple of applications then you probably have a vacuum leak, be it the booster it's self, the vacuum hose or the check valve.

If all those checks are ok, then it should mean everything is functioning as intended.

As streetwise guy suggested, if your old car had the same pedal travel engine running or not, then you most likely had a defective booster.

If it is the FC RX-7 that is in your avatar, the push rod is adjustable. You should be able to remove the nuts for the master cylinder and pull it away from the booster enough to adjust it without cracking the brake lines.

Thanks for the replies, all the functionality tests confirm the parts are all working. I was comparing the two cars but now believe that it is a good possibility that the old car never had a properly working booster. I have not had a chance to crawl under the dashboard yet to see if I can adjust the pedal a little to help them on initial bite. That and I will just need to get used to the new pedal action.

The pedal travel should be adjustable from the engine side of the car. Pull the master from the booster and then see if there push rod has an adjustment.

I'm guessing you've gotten this from context, but just to be explicit about a couple of items:

1) The longer travel when the booster is working is just a byproduct of the booster helping you push harder. So it's the same travel per force, it's just that for as hard as you push, the booster is helping you push harder, so all the compliant bits in the system move a bit more, and you get more travel.

2) Pushrod adjustment can be fiddly; go a little too aggressive in making it longer, and you can prevent the master from returning all the way and opening to the reservoir. This causes two problems, which are that the master can't replenish itself (or bleed properly). Also, it can't adjust as the fluid warms, so you can get the system locking up as the fluid trapped between the calipers and the closed master expands. Which of course feeds back as it causes increasing brake drag... You see where this is going

In reply to imgon:

Good to hear things are working correctly. Provided the brakes are performing correctly when you're driving, I'd take a bit of time getting used to it before you go trying​ to adjust the push rod, you could cause yourself more headaches than you need.

In reply to Ransom and daeman:

Thanks, at this point I think I will just leave everything alone. I have two test and tune days coming up and that should give me a chance to get used to the new 'feel" of the brakes. The event I went to last fall I was pumping the brakes as I was unsure what was going on, that was obviously costing me alot of time. I was just so used to the brake pedal being way up high. Once I get used to the new height it may help me with heel/toe as the pedals will be closer. I'll report back at the end of the week with the results.

So after two events and close to 300 miles I still am not happy with the pedal travel. I do feel like I am getting used to it but would like to improve the length of travel. I did adjust the pushrod about a 1/4" (about half the available length) between events but it did not appear to change anything. The brakes do work very well, it just takes a while for the pedal to engage. Based on measuring the vehicles I have it seems that the older/ higher mileage the car is, the more pedal travel there is. Is this just a by product of an old master cylinder? Is there a way to get the pedal travel back to "new car" levels? Thoughts??

An older master cylinder can feel a little sluggish I guess. As they get older, the seals wear and harden, the bore can also wear and get small scratches. This could contribute to pedal feel a little. It'd allow some fluid bypass to occur briefly before the pressure was enough to help form a positive seal.

Also have a good look around the firewall where the cylinder and booster mounts. Repeated flexing of the fire wall can lead to cracks Wich will make for a saggy feeling pedal. Likewise, check the pivot bushings for the pedal and make sure nothing is flogged out. Whilst on the topic of fire wall flex, there are master cylinder braces available. They usually bolt to the strut tower or inner guard and push against the end of the outside of the master cylinder​ to help limit firewall flex.

Another possibility is your soft lines, over time, the rubber degrades and they can begin to balloon slightly when under pressure, which will contribute to a slightly spongey feel. I've found braided brake lines have given me a firmer pedal feel in the past, so may be an upgrade worth considering for you.

Outside of that, have you upgraded calipers or anything like that? A larger piston or multi piston as opposed to single piston caliper than was originally fitted will cause more pedal travel if you've retained the original master cylinder. Generally, bigger brakes require a larger diameter master cylinder to move sufficient fluid.

The only other thing I can think of worth considering would be having your booster checked out. A slightly perforated diaphragm or worn seals or reaction disc could have a negative effect on pedal feel.

Use a dial indicator to check for any runout in the rotors. You could be getting some pad knock back, and the initial pedal travel is being used to get the pads back into contact. With the car up on jacks, apply the brakes real hard. Then get out and spin the wheels 20 or 30 revolutions. Take the wheels off and check and see if the pads are tight or floppy against the rotors. If they are floppy, you have an issue that can be solved with 2lb residual valves. If you need a dial indicator to check runout, stop by my house and I'll loan you one with a magnetic base. You can stick it to a strut and spin the hubs to see if there is any wobble. Double check the wheel bearings to make sure they are tight as well. May not solve your issue, but at least its something you can check.

the replies above have covered everything I came here to say.

Thanks for the suggestions. The pedal is firm once it engages, just takes too long to get there in my opinion. One thing popped from daeman's post. The car is a base model that originally had the 4 lug single piston calipers and I am running the 5 lug 4 piston fronts and single piston rears. The old car was a similar swap but I bought it that way and wonder if they had switched out the MC. I will look into that as everything else passes the "check this" tests. I'm reasonably sure the calipers, pads and rotors are all doing their jobs properly as I was using the parts swapped off the old car last fall and installed new pads and rotors this spring and had no change in pedal travel. I am going to be talking to the guy that help build my original car later today and will have to see if he remembers/know if they replaced the master cylinder and will also check the parts store to confirm they use a different one. This could be the answer. Thanks again, this forum rules.

In reply to imgon:

Given what you just said, I'd almost be willing to bet on your master cylinder being undersized for the upgraded brakes. The symptoms match perfectly. I'm pretty sure you'll find your current master is a 5/8" bore, where as a 7/8" bore would be much more effective with the 4 spots up front. Keep us posted with what you find.

It turns out that Mazda uses the same MC for both brake systems (2 or 4 piston) it offers in the FC. Based on that fact, it seems I either have a problem with a caliper releasing too much or just a sloppy pedal. I haven't had a chance to get the car out of the trailer yet to see if it is caliper related or maybe firewall flex related. I guess the good thing is I keep narrowing down the potential problem with out blowing money on parts I didn't need. Thanks again for everyone's ideas/suggestions.

So here I am a year later and the brakes are worse than they were last spring. I tried all last season "to get used to it" because I could not find any parts that were not working as designed. Everyone said to expect extra pedal travel because power brakes. Once the pedal engages the brakes, they work awesome but there is almost 3" of pedal travel to get there. So over this off season  I have replaced the master cylinder, booster, pads, rear calipers and rotors. System has been bleed about 4 times due to all the components that were replaced. My bleeding procedure is to vacuum bleed all four corners typically passing about 4-6 ounces of fluid through the lines. Then do a pedal pump bleed. This results in an awesome pedal feel, about an inch and a half of pedal travel until you can't push the pedal any further. Then once the car is started the pedal  goes  most of the way to the floor before it engages. This morning on it's  maiden voyage I tried  disconnecting the vacuum line to the booster and it did what I expected,  the pedal didn't travel very far but I had no stopping power. One thing that was not expected was when I disconnected the vacuum line the idle improved greatly. Typically the car just barely  idles at about 800 RPM and sounds like it is about to stall. With the line off it jumped up to about 1000 RPM and seemed much smoother. I have another RX7 that has been sitting for 2 years, has brake fluid that is who knows how old and the pedal only travels about half as far before engaging. WTF am I missing?  

I was just re-reading the thread, and I was really expecting to say "okay, it's time to adjust the pushrod", not expecting to find that you already had, and that it didn't seem to change anything.

If you can adjust the pushrod and it doesn't change anything, something's funny.

You start pushing the pedal. The arm is attached to the pedal and moves. The pushrod is moved by the arm (or the arm and some linkage), and it drives the first piston in the master cylinder. If you can change the length of something between the pedal and the piston and it doesn't change what's happening, then something is wrong, I think.

I find myself wondering whether there was a friction cap on the pushrod that left with the old MC and didn't get added to the new MC or something. I'm trying to reason out where you could make a change and not have it affect where the system finishes resolving all its mechanical and hydraulic slack.

Okay, here's an out-on-a-limb question: Where did you make the pushrod adjustment? I'm thinking of a pushrod that should actually reach into the master cylinder, and be the very last mechanical item in the linkage before the hydraulic bits. I believe some cars have some adjustment down there that would affect pedal height (I think), but *wouldn't* affect the slack in the system.

To rephrase, there's a sort of "neutral" position of the pedal when you're off the brakes. The pushrod we want to adjust would take up slack from that position between the pedal and the master cylinder. The other adjustment would probably adjust where the pedal physically sits when it's in its neutral position, but would not take up any slack between pedal and master.

Does that make sense? Got a photo or description of where you made the adjustment?

I know you've been through every part of this system several times, so apologies for what may be a "have you tried rebooting it? Is it plugged in?" kind of question...

The only adjustable part is the rod from the pedal to the booster. I presently have it as long as possible.  My thinking for that was I want the rods to push on the MC as soon as possible, do I have that backwards? If the pedal always travelled a long a distance I could totally understand some extra travel under power assist but to have such a drastic difference between boost/ no boost baffles me. I really thought with the new MC and booster the issue would at least get a little better but I think there is actually a little more travel than the 30 year old  parts.

One other tidbit of information , don't think it makes much difference as the problem is the same with either set up. There are two different boosters available for the car, one is slightly bigger than the other. The original one was the smaller version,  the car originally had the single piston calipers.  When I ordered the new one I got the larger version designed for the four piston calipers.  This is the same booster as the other car I have with the better brakes. 

I don't know if the vacuum pressure could the problem as I have never checked these values.  The vacuum line is connected to the same place it has always been, the engine has been in both cars. But yesterday's idle change with the hose disconnected got me thinking about pressure not parts. I will see if anyone I know has a vacuum gauge and see if that is out of spec.

• Vacuum: when you disconnected it, did you plug the side going to the intake? If you did, then it was probably running worse when connected due to a leak in or on the way to the booster. If you didn't plug it, then you basically were giving it a bunch of extra unmetered air.
• More travel with boost: this has to be about compliance. Whether it's air in the system, flexible lines, or bending metal, there is no change in the stack of metal and hydraulic fluid between your foot and the brake pads based on vacuum. Barring grassy-knoll level conspiracy theories about the booster being slightly engaged with your foot off the brake, and thus causing the master/pushrod never to return to rest with the engine running, but I suspect that way madness lies.
• Pushrod: I'm still baffled by the notion that you're not seeing changes when you adjust this. You are basically correct about wanting to make it longer to raise the engagement point, BUT I have a caveat: if you make it too long, it can prevent the master from releasing fully, and that will prevent the rear circuit from replenishing from the reservoir or really bleeding properly.

At this point maybe I would be doing something wacky, like... Disconnect the hard lines and make sure the master doesn't push itself away from the booster as you back off its nuts. Or even try it without disconnecting the hard lines in case it's obvious? Or... Make a telescoping placeholder to put in for the pushrod and see what the neutral length is? That's a little fraught with the shapes that might be involved...

Vacuum- when I removed it from the booster the idle improved, I plugged the hose with a bolt and a clamp and the idle returned to just barely  running,  normal, for this car. With the booster disconnected the pedal had very little travel but almost no braking  ability.  This is what I expected. 

With the booster reconnected,  back to 3" of travel before brakes engage. With the engine off the pedal only travels about an inch and a half and is very firm. If there was air in the line the pedal should be mushy, correct? Before swapping out the MC and booster had a friend check for firewall flex, there was a slight amount of flex but not really measurable, you could just see a little movement in the firewall. The brakes do not seem to be hung up with the foot off the brakes.

Pushrod-I checked the other car I have and the jam nut is in almost the same place. Do you think it would be worth trying adjusting it all the way in to see what happens. Last year I tried moving it about half of what was available ( about half an inch) . If I adjust it all the way in it should have an even longer travel before it engages the brakes?? Again before the MC /booster swap I had my friend work the pedal (both running and not running) and while running the brakes didn't start engaging until pedal was about 1.5" to 2". When not running it took about an inch of travel.

I'll try loosening the MC nuts and see what happens.  Your thought is that if pushrod adjustment is off, the the MC will be under pressure even with pedal not engaged? Will need the car running for this test, correct?

Still need to find a vacuum tester to see if there is some kind of leak. Would a leak make the pedal travel longer?

I'd have to take a tape measure out there, but 1.5" is striking me as a lot of travel for without boost.

The pedal could be remarkably firm off boost without being properly bled. The whole system is sized for having boost, so you don't have as much mechanical advantage. Moreover, when you're going back and forth, it's going to give a much firmer impression than if you were just trying it by itself. Question: Without boost, is it 1.5" free play then firm, or does it have rising resistance over that travel? This would be so hard to judge that it may be useless, but basically I'm thinking that if it's totally free then wham, hard, that would indicate slack somewhere in the system between pedal and MC. If it ramps up, that would indicate air in the system. Again, there's a spring on the pedal, it's super-subjective... Probably not meaningful.

The firewall being reasonable makes sense, as the booster isn't adding any force to flexing the firewall, so it'd be weird for extra travel to be coming from there.

Especially given how weird this car is being, I don't think you can compare one pushrod to another and say that "same" equals "correct". I got some Rabbit MCs that changed the shape of the piston where the pushrod engages; I swapped out a couple before giving up and deciding that something had changed (maybe my car had the wrong MC on it before), but I ended up having to reshape the acorn nut on the end of the pushrod because the well in the MC was too small for the nut's original shape, and it hung on the points of the nut instead of riding on the end.

Regarding loosening the MC nuts, yes, I'm wondering whether it's staying slightly engaged. You do not need to have the car running; you should either be able to observe that the MC's spring ejects the pushrod and pushes the MC away from the booster as you loosen the nuts. Note that this may be tricky thanks to the hard lines trying to hold it in place if you have them connected.

It's fiddly stuff where small measurements can make a big difference, but if you end up with the MC off again, you might try sticking the pushrod into the MC off the car so you can measure/mark the point on the pushrod that should be even with the mounting face of the booster, then see if that appears to line up when you put the pushrod back in and put a straightedge across the booster next to it.

Is it possible you have a pedal from the manual brake version of the car? the pedal ratio is different.  You may just need to move the attachment point so the rod travels further per the pedal travel.  Some MFG put both holes in the pedal some don't.

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