Just a shot in the dark to see if anyone can help my mechanic friend with an ongoing brake problem he has had for literally years on his '76 T/A . He doesn't subscribe to any message boards, and has asked me to relay his info. I am pastiing what he wrote below, in hopes that somebody out there has experienced the same problems and might have a suggestion for him.

Thanks in advance. . .
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I have a problem with the brakes on my 76 Trans Am that is driving me nuts! Basically the problem seems to be a lack of pressure to the front disks. You can barely lock the front disks under any circumstances. A severe panic stop will lock the rears and can possibly make the car want to come around on you. This has been an ongoing problem I have been toying with for many years, (I've had this car for over 15 years) and can't seem to whip. Hopefully someone else has seen this same thing before and has cured it. It must not be too uncommon, because I borrowed another one of these Firebirds and found the brakes to be about the same. They work but just don't feel quite right or stop properly.
I will try to explain.
The car requires way too much pedal pressure to stop it. You can actually distort the firewall with the pedal pressure. The car is quite drivable but just not right. I am positive the clamping force of the front brakes is inadequate because after every part in the system was replaced, (and I mean every part except the steel lines, which were flushed and flow clean and free, and seem to be free of any dents) I borrowed an expensive set of gauges that are placed between the removed front calipers and read the actual clamping force generated at the disks. We backed this up by taking my sons 75 Grand Prix which used the same components for front brakes exactly (except it stopped properly) jacked them up side by side and ran the same test one right after the other. On the Grand Prix, moderate pedal pressure with the car running generated 5000 pounds force, while on the Trans Am standing on the pedal with both feet could only make 3500 pounds force. At least this proved I was not nuts!
This car does not see many miles. It has a fresh 440 inch 4 bolt 455, ( .040 428 pistons, 421 armasteel crank combination) with an 068 "s" cam, and makes good vacuum for the booster operation).
During the years of ongoing testing, trying to take all the possibilities out one at a time included------
New front and rear hoses.
Rebuilt front calipers.
Brand new rotors.
3 different rebuilt master cylinders.
2 power boosters. (all of which passed the GM function test)
2, one used, one brand new from GM 3 way combination valves. (3 function hold off-balance-proportioning valves)
Brand new front pads, (three brands and compositions)
New rear wheel cylinders.
New rear shoes.
New rear drums.
Master cylinders were bench bled and bled on car.
The car has been pedal bled in several different fashions, right rear to left front, and one rear and one front at a time to try to keep one side of the system from completely letting the cylinder bottom.
The car has been vacuum bled.
The car has been pressure bled, with and without the factory tool which holds the combination valve released.
The car has been bled while tapping on the calipers to free any sticky air bubbles.
The car has been bled flat and with the rear in the air till the cylinder was level. (which requires the rear about 5 feet in the air).
The car has been bled with the Phoenix injector.
The car has been reverse bled with the Phoenix injector.
The car has been cross bled with the Phoenix injector.
The car has been bled with the pedal depressed one inch to close the return port as the Phoenix video suggested.
This system has been bled so many times about 8 gallons of fluid have passed through it.
Multiples of these tests have been tried as backup. Other things have been tried I can't even remember! Totally off the wall things have been tried as well.
The pedal motion is very short, (normal, good pedal) every time it's finished.
Needless to say this system is clean as anything out there.
The car does not pull left or right and never has.
The button on the combination valve moves in and out as normal.
This is a rust free Nevada car and has never been farther away than California, so corroded parts are not a problem.
I'm at the end of my rope!
I have worked on cars for almost 40 years, was GM certified Pontiac, Cadillac, and Frigidare in the 60's and 70's, factory certified by Honda and several others, was a service manager for 8 years, so I think kind of know my way around this stuff, but I'm sure I'm still missing something. Parts were changed multiple times to try and avoid the "bad part" syndrome. I have several friends still in the automotive trade, one who even teaches for one of the trade schools, and so far nobody has thought of anything else to try.
I believe somebody out there has run into this and solved it before!
I'm looking for that person!
It has to be something basic or stupid or overlooked as most problems turn out to be in the end.
Thanks for any suggestions or questions.

 

WOW!

Your friend has tried about everything.

The only thing that occurs to me is a brake pedal problem:

1. improper pushrod length: if for some reason the pushrod was wrong, things would bottom out before the master cylinder stroked far enough to get full pressure.

I don't think your list said the pushrod was ever changed.

You might check that.

2. a long shot: binding or restriction in pedal travel

3. a longer shot: the whole pedal mechanism was changed and the pedal ratio is wrong, or it's causing the same thing as a short pushrod.


If I remember correctly, 100 lbs. pedal force should be the maximum needed to get full braking force.

Good luck!

My $.02

 

If I was him I would have bought an adjustable proportioning valve a long time ago. That way he can dial in the brake force from front to rear however he likes.

 

Since he tried another one and felt that it was the same, I’d be looking at a few things in the basid design (sure doesn’t sound like he’s got something broken/worn out with what he’s tried). First, he said that he compared it to a Grand Prix that used the same brakes, and it was fine. Did it have the same MC? What about the pedal ratio (the ratio of the distance from the pivot to the pushrod attachment relative to the distance from the pivot to the pedal). I suspect that he’ll find that the grand prix has either a different MC or a shorter pedal ratio, and if he’s happy with the way that is working then I would try to duplicate it. Also, if it has similar rear brakes he’ll probably need either the combination valve out of a Grand Prix to keep the rears from locking, or just install an adjustable proportioning valve and set it to anything that he wants.