I have been working on installing a GN drivetrain into an 87 Ford Thunderbird Turbocoupe. (some pictures at
http://www.angelfire.com/bug/thunderbirds). I know my brake problem is with ford parts but brakes are brakes, and the ford guys are stumped.
The Situation
I have an 87 Ford Thunderbird Chasis that has had the ABS removed and rear disk brakes installed in place of drum.
Problem
- Rear brakes only give 5-10% of what they should. (When car is jacked up, a tap of the throttle will spin the rear tires with full brake pedal applied)
Set up
- 1988 Mustang 5L brake booster (double diaphragm)
- 1989 Mustang GT Master Cylinder
- Aftermarket adjustable pressure valve to rear brakes
- Direct brake line to front brakes
- Brake pedal from the Manual Transmission turbocoups (There are a couple of different styles of brake pedals, I have the one that gives you more leverage)
What I have Tried / Discovered
- I tried opening the pressure valve to allow full pressure to the rear brakes. Adjusting this valve would give me 1-5% of proper braking.
- If you pump the brake pedal hard four consecutive times holding the pedal down firmly on the last pump, the pedal should not drop and remain hard for as long as you hold it. What happens is the pedal will get harder as you pump it but will not remain fully extended. It will fall a good 3/8 – 1/2 of the way down before holding. ( I just replaced the brake booster with a used one that was supposed to be good, but who knows)
- I adjusted the rod that comes out of the brake booster to the correct length for the master cylinder. I extended the arm out a few turns until the front brakes started to come on with no brake pedal applied and then backed it off slightly. This helped the rear brakes. Instead of being around 5% of what they should be they are now 10%. Still not at all adequate. (I also tried lengthening the rod a lot more just to see what would happen. If it made the rear brakes better it was only slight)
- I know there is movement in the brake calipers, I had someone look at them and they are fine.
What I am thinking of trying
- I am thinking of undoing the front brake line from the master cylinder. There might be a chance that the front brake lines are hitting full pressure, stopping the cylinder inside the master cylinder from extending forwards before it can apply enough pressure to the rear.
- First attaching a pressure gauge at the rear brakes to see what pressure they are getting. If pressure is fine at rear then attaching gauge at the master cylinder. Does anyone know what the pressure should be at both locations?
There is a chance that the master cylinder that is currently on the car is for rear drums. Would this explain the brake pedal not becoming as stiff as it should when you pump it?
Any help on solving this would be greatly appreciated.