Gotta fever, I need more brake pedal

What it is:
1993 F150 with The "Factory Big Brake Kit" on it, which is a 2004-2007ish E150 front spindle and brake setup

https://www.nloc.net/forum/index.php?threads/factory-big-brake-kit.210296/

The NLOC forum advises a '99 Expedition master cylinder, and that's what I went with.  Also have 2001 Lightning wheels on the truck, not that I think that's contributing to the problem, but more information is always good and they are a little bit more rotating mass to resist.

Problem I'm having:
Especially with towing, I feel like I'm not getting enough brakes.  It feels less competent than with the stock brakes.  When she was stock I was always able to lock the brakes, towing, loaded, unloaded, whatever.  Now no matter how hard I push I don't get that feeling.  I worry that I wouldn't be able to overpower the engine.

Contributing factors:
Brake system is in good repair, stainless steel lines about a year before I got the brake upgrade.  However, the brake booster is (very possibly) stock at 400k+ miles on it.  I know that in the 260k+ miles and 20 years I've had the truck I haven't replaced the booster.  I've also got a misfire/stumble that I've been trying to track down, so it might be the booster?

My main concern right now is the hydraulics.  Just because the guys on NLOC are happy running the Expedition MC doesn't mean its correct.

Could it just be the pads?  I know my Silverado seems pretty picky about brake pads.  It seems the OEM is the only one that gives me the feel that I like.

You can get a brake pressure testing kit that screws into the bleeder ports.  That should be able to tell you how much hydraulic pressure you are creating at the calipers.  Perhaps Angry can give you an idea of what that pressure should be.

In other words, I think the brake pressure kit will tell you if your hydraulics are working the way you want them to.

Does your truck have drums and the donor have disc rears? I've seen mc oversizing due to that a few times. Other obvious one is just a little residual air after the swap. 

When I did the brake conversion on SanFord, I used a site like this. 

https://motionraceworks.com/pages/brake-system-setup-and-calculations?srsltid=AfmBOooW0IZfRQ1loYU4lZkDXWU67v6k2Rbi258tYe0-kY5Ajt9ewFby

You want 1200 psi at the caliper with 100+- pounds of pedal force. 

Boosted systems use around a 4:1 pedal ratio.

Then you need to figure out the area of the master and the area of the wheel cylinders/calipers. From there, it's figuring out how much fluid the master puts out and how much fluid the wheel cylinders and calipers need. 

If you don't get your 1200 pounds the brakes are going to feel weak. 

Rock auto shows the donor for the brake kit has 1.125" master cylinder and the Expedition has 1.25".

That .125" smaller will help your pedal feel a lot.

Vacuum leak on the booster? Could just be a small crack on the hose.

Is the truck disc/disc or disc/drum?  I have a 1991 F250 and unless I have the rear shoes adjusted correctly the pedal is soft.  And that means backing off the parking brake adjustment, and adjusting the shoes.  Then re-adjust the parking brake.

Most of the queries I have are asked above, but does the pedal feel soft? Hard? Normal?

Toyman! said:

You want 1200 psi at the caliper with 100+- pounds of pedal force.

as the girl says to Meat Loaf, stop right there!

you want as much pressure as it takes to lock the wheels with a reasonable pedal effort when your vehicle is at GVW. There is nothing magical about 1200 psi.

If your apply system (pedal -> booster -> MC) can't generate that pressure, then you either need more pedal effort, more boost, smaller-bore MC, or more brake corner output.

Brake corner output comes from caliper clamp force, pad friction, and rotor effective radius.

There are trade-offs to every one of these possible changes.

Higher pedal ratio means more pedal travel. Will the pedal hit the floor before max pressure is achieved?

Higher boost ratio will hit runout sooner.

Smaller MC bore will require longer pedal travel to move the amount of fluid the brake corners need.

Larger caliper pistons require more fluid to clamp.

Higher pad friction may come with more brake dust, more and/or faster rotor wear, and/or more noise.

Larger rotor diameter requires new caliper brackets and maybe won't package in existing wheels.

Of all these, increased pad friction is the easiest to implement.

Paul_VR6 (Forum Supporter) said:

Does your truck have drums and the donor have disc rears? I've seen mc oversizing due to that a few times. Other obvious one is just a little residual air after the swap. 

It could be oversizing. Going through my notes I went with the 1-1/4" bore MC.

Bled them a few times to ensure they were clear.

theruleslawyer said:

Vacuum leak on the booster? Could just be a small crack on the hose.

May be the easiest to check out.  I'll try to do that tonight in between packing

Jesse Ransom said:

Most of the queries I have are asked above, but does the pedal feel soft? Hard? Normal?

Pedal is very firm.  Does not feel like air in the lines, but also doesn't have that give the booster normally has, if that makes sense?

I have yet to pump the brakes while engine off, then crank it and see of the pedal moves

In reply to AngryCorvair (Forum Supporter) :

Based on what's in here and the answers, do you have any suggestions?

Fairly sure I went with one of the higher end NAPA brake pads when I did this swap.

If the pedal is very firm that might mean you're not getting much assist, er, boost.

boosters mostly fail internally since they're basically a big rubber diaphragm inside I think.

1.125" dia area ~= 0.99 in sq
1.25" dia area ~= 1.23 in sq

That's approximately a 20% drop in line pressure for a given input pressure for the 1 1/4" master when compared to the 1 1/8" master. (unit pressure over 1.23 compared to over 0.99)

A firm pedal that feels like you're not getting the expected results sounds like that kind of ballpark.

I think you're right to sanity-check the booster. But the larger master piston will also reduce the amount of "boost-assist squish" because it's reducing the hydraulic force on all the bits that normally give.

I'm curious what Angry says, though.

Do we know anything about the piston areas on the original and upgraded calipers?

In reply to Jesse Ransom :

Going off memory, twin 2-1/8" pistons on the new ones. Single 2-13/16" on the original?  Rotor OD increaes by about an inch?

Okay, so the new piston area is about 7.1 in sq, and the original piston area is about 6.2 in sq...

New calipers:
2.125/2 = 1.0625
1.0625^2 ~= 1.129
1.129 * pi ~= 3.55 in sq
times two pistons is 7.1 in sq

Old calipers:
2.8125/2 ~= 1.41
1.41^2 ~= 1.98
1.98 * pi ~= 6.21 in sq

7.1/6.21 ~= 1.14

So you've gone up 14% in caliper area, and appx 24% in MC area; I feel like I could at any second do a less than percentage vs greater than percentage mistake, but I think your master cylinder input shaft force would need to go up... 124/114 ~= 1.09... about 9% to get the same braking at the front calipers (before we factor in the increased torque from the larger rotors).

In reply to Jesse Ransom :

Hrm.  And 1.25/1.125 is 111%...  might be the answer, long term?

In reply to Mr_Asa :

Don't forget that you've gotta do the area math for circles. 1.25 is 111% of 1.125, but the proportionality is on the square of the radius. That's where that 0.99 square inches vs 1.24 square inches came from.

Right now it takes an extra 9% force at the master pushrod; if you go back to the 1.125 master then you'd get a reduction in that force proportional to the increase in caliper area. 1/1.14 ~= 0.88, so about 12% less pressure than stock calipers. With a side order of more pedal travel (12% +/- linkage angles etc?)

Er, wait... Was the 1.125" master stock or just one of the upgrade options? The above assumes 1.125 was stock.

In reply to Jesse Ransom :

Stock master is 1.0625"/27mm. So it is worse than your calcs show.

In reply to Jesse Ransom :

There's an Expedition MC at 1-1/8. 

Jesse's math is correct.

Not sure what he means by "boost-assist squish".

Pads have a two-letter code to identify their friction output at "cold" and "hot" conditions. This article describes them pretty well:

Disc brake pad edge codes

From that article:

In the edge code, the first letter denotes the normal coefficient of friction (temperatures ranging from 200° F to 400° F) and the second letter indicates the hot coefficient of friction (ranging from 300° F to 650° F).
 

And from the SAE standard which defines the letter codes (screen shot from an article at hotrod.com):

I can't make a specific recommendation because I have no personal experience with these trucks. I'd look for something with FF, FG, or GG friction code.

AngryCorvair (Forum Supporter) said:

Not sure what he means by "boost-assist squish".

I should not have thrown a random made-up phrase into a discussion that already has so much opportunity for confusion.

It was an attempt to capture that softening of the pedal one notices if the booster has no vacuum and then you start the car and the booster's assistance makes the pedal much softer and longer, and that given that this is the result of compliance in the system, anything (like a smaller MC) that increases line pressure for a given pedal effort will also increase that compliance (or lessen it for a larger MC).

It really just muddies the waters and I should have left it out.

jgrewe said:

In reply to Jesse Ransom :

Stock master is 1.0625"/27mm. So it is worse than your calcs show.

Okay... So we have:

• Stock MC area about 0.89 in sq
• One size up Expedition 1.125 MC area about 0.99 in sq
• Currently installed 1.25 MC area about 1.24 in sq

And for calipers we have

• Stock 6.21 in sq
• Upgrade 7.1 in sq

So the original stock ratio of MC area to caliper area was 0.89/6.21 = 0.14

And the new current system is 1.24/7.1 = 0.17

So the *front* calipers are going to need to see 0.17/0.14 ~= 1.2 times as much input force to see the same clamping force
The *rear* brakes (calipers?) are going to need to see 1.24/0.99 ~= 1.25 times as much input force to see the same clamping force (because they're having their hydraulic pressure reduced by the larger MC but didn't gain any piston area themselves to offset this. The front calc is based on the changes to the relative sizes of the MC and caliper, the rears based on the change to the MC with the unknown caliper held constant).

So you've increased the effort on the front brakes and simultaneously reduced the contribution of the rear brakes by a larger proportion (of a smaller contribution).

If you go to the 1.125" MC, the new MC to caliper ratio will be 0.99/7.1 ~= 0.14 (rounded to two places), so the fronts will go back to effectively the same ratio as stock, but you'll still have a drop in the contribution of the rear brakes due to the larger MC (but considerably less of a drop than with the 1.25" MC).

This still ignores the difference in torque of the larger front discs, and we still haven't talked about what your rear brakes actually area (disc? drum? Is there a proportioning valve? I honestly haven't a clue how linear those are and what happens when you move the range of line pressures up and down...).