Track Car 1987 RX7 Brake problem

I am looking for help here.  Long story, please hang on for the question...

My 1987 RX7 Chumpcar got more horsepower this year.  It is now 10 to 15 mph faster at the end of the straights.

Problem: I am destroying front and rear brake pads and boiling Castrol SRF fluid within about 6 hours.

Background: For the past 5 years (before the horsepower increase) we always ran stock brakes with Hawk Blue Front and Hawk HP+ rear pads.  They lasted for full 24 hour races with no swapping needed.  We had to replace the master cylinder after Watkins Glen because the fluid was leaking from the reservoir.  Note:  I did push brand new clean fluid from the all 4 calipers into the master when I was bleeding them...  Is this the issue?

Attempted fixes:

1) New pads (went to DTC 60) - no difference - boiled fluid, pads destroyed

2) Brake ducts - forced air right to the center of the rotors - minimal to no difference - boiled fluid, pads had a sliver of material left but did NOT stop the car.

3) Different new pads(ST43), new rotors, new hoses, new ducts (with blowers) - front pads are 1/2 worn at least (pads are reported to last 50+ hours)  Rear pads are toast!

Notes:

1) I checked the plunge of the brake booster rod into the brake master and it measures fine.

2) Other teams are running the same / more HP and speed in this same chassis and are not having this issue even with similar or lesser pads.

3) I am going to look for damaged hardlines (to see if they are holding pressure somehow). 

Any other ideas?

Thanks,

Rob R.

You've got a better brake setup than the average low-budget race car already, so something's very wrong here, and it's wrong at all four wheels - it's almost certainly a brake drag problem. Pull off the track with fully hot brakes, get a jack under the car real quick and get someone with oven mitts to try turning some wheels by hand. A little drag can build up a lot of extra heat.

In reply to GameboyRMH :

I agree that this is probably a brake drag issue. 

If we go on the assumption that it IS a brake drag issue, what do I do about it?  Where should I look?

I already flushed the system, installed new hoses, rebuilt all the calipers (forgot to include that in the first post).

I would check the hard lines and for any mis-specified components...it sounds like your system is mostly stock though. You say this started after you swapped the MC?

GameboyRMH said:

I would check the hard lines and for any mis-specified components...it sounds like your system is mostly stock though. You say this started after you swapped the MC?

Yep.  I am heavily leaning towards this as the issue.  However, I would prefer to find some way to validate the issue (or definitevly FIND the problem).

I am going to tear the "new" master cylinder apart and see if there is anything to see.  I will compare that to the original master cylinder that came off the car.

Thanks for your help!

If that doesn't work well then maybe its time you put the front brakes on the back! I was always curious if this could be done easily or not... it would be a HUGE amount of braking but your proportioning valve may need some adjustment.

Sounds like too much friction…when was the last time you oiled your brakes?

Since you checked the booster push rod for free play I would have to say that you should try another master.

I'm not sure of the cost but most masters are not that expensive so I'd start with that.

Any changes in the driver lineup?

Is it possible that the driver is riding the brake pedal or excessive braking not giving time to cool.

I know driver error is not easily discussed .

Rob,

Off topic, but are you still interested in the wheels off of your old car? Got my new shoes mounted a couple weeks ago and I've been meaning to get a hold of you about them. Might be out your way in a few weeks anyway and could bring them along.  

YES!

Shoot me an email and we can discuss specifics...

wvumtnbkr at hot mail.com

There are only two possibilities here (well at least in my mind)

1. The replacement master cylinder is wrong; somehow you have a master that was from an earlier disc drum car or the piston height is different (it's closer to the booster so although the pushrod  length is fine it still has residual pressure)

2. The push rod either at the pedal or at the master is maladjusted or is somehow positioned wrong. 

I would jack the car up, spin the wheels to ensure they do not require a ton of force to turn them. If they're free then squeeze the brakes a couple of times then let them off slow. Check the wheels. If free run the car in gear (this will test the back) do same wheel check.

I find it unlikely that  a line suddenly went bad. It's possible but unlikely.

Email sent.

So, the driver lineup is all people that have been in this car and raced before.  I don't suspect that as the issue.  This happened over 4 separate race days with different drivers from the same pool as well.

I took the new master apart and found nothing abnormal.  I will take the old master apart and see what it looks like in comparison.

Are you using the turbo brakes and 5 lug setup, just wondering I can't comment on how to help you.

The turbo brakes are the same as the GTU brakes, or the GXL brakes, or the Sport brakes, or the "vert" brakes on an S4.  The S5 had the good brakes on the Turbo and GTUs. 

We are using the 5 bolt brakes which are the "turbo" brakes. 

The only difference is the rear rotor is vented on the turbo cars (we are running the vented rear rotor).

Rob R.

I vote for a wrong MC or a defect in it. What about the brake pedal return spring? 

I had an issue on a Rabbit at one point where the *shape* inside the replacement M/C was different, and the acorn-nut type pushrod end which interfaced with the M/C didn't reach the bottom of the bore, so even though the overall length was right, the corners of the nut were contacting the taper of the bore, causing it to act like it was longer than it actually was, with predicable results. I'm not sure exactly how you measured the plunge of the rod into the M/C, so you may already have ruled this out, but tuck away the general possibility of *really weird* old car bits...

Simplest explanation points at M/C, but this is a car, and they find interesting ways to go wrong. I don't know where an RX7 M/C falls price wise, but given the difficulty of troubleshooting something like this, I'd be inclined to try another one if you can't isolate and resolve the problem definitively. Whether it's an internal problem with the M/C, or something funny about it that makes it not work with your car correctly, it's the landslide front-runner for "likely culprit" since it's central to the symptom and changed right before the problem.

Agreed.

Here is my problem:  I want to determine what is wrong and solve it through some type of measurement. 

I can not drive the car on the street.  Even if I could, there is no way to get the brake pads up to proper temp without being a nutball.  Therefore, I would need to spend money on a trackday or something.  The issue with this is that it is VERY difficult to tell if the car has an issue until a few hours are spent on track.

Therefore, I am looking at ways to measure the system.

Right now, i am going to order 2 brake pressure gauges with some AN3 hoses.  I plan to hook the pressure gauges up to the rear brakes, the front brakes, and one front and one rear to see what sort of differences I am getting.

If this doesn't tell me anything, I am probably going to replace the master (again) and get new XP24 front and XP10 rear pads.

 I had this same issue and it occurred when I swapped in the brake system from an 87 T2 into my 86 along with the hubs. There is definitely a difference in the master cylinders and possibly the booster. It f I remember correctly, and this was at least 15 years ago we’re talking here, I had to trim the rod that pushes on the master to make it work properly it sounds like this is probably your problem. 

I dig it. Seems like without a pressure reservoir, pressure should drop to 0 pretty quickly if the M/C is behaving itself.

Getting the gauges inline in the actual system sounds like a pain, but will tell you things about bleed-down from operating pressure.

I was half-wondering about using a hand pump and gauge on an extension plugged into the bleeder hole to simply gauge whether applying a small amount of pressure bled down to 0 quickly without having to start breaking into the rest of the system.

Yep, the gauges are plumbed into the bleeder hole.

Like dis one

However, I am going to make some hoses and add a bleeder to the system as well so i can make sure there is no air in the gauge.

I suspect you won't have to drive the car at all to find the problem. About 5 years ago my Datsun had a sticking master or so I thought; I'd swapped out to a larger master and there was an ever so slight difference. The net effect is the brake pedal return stop was adjusted to tight and the the piston wasn't fully returning, we're talking .010 if that 

If I depressed the brake pedal and then tried to spin the wheels the would be noticably harder to spin, say maybe 10-20 pounds of force but after a few spins (3 or 4 rotations) they'd free up, not completely but say down to 5lbs of force. 

You probably don't even need to jack the car up, squeeze the pedal and then see if the car rolls easily when you push it. If it's hard to push back off the nuts on the master by half a thread or so see if if that solve it.