Brake Problem

[Don't Panic]

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.

[KWIKR 1]

I'm not sure what the plumbing on those MC look like but ,any chance you can reverse the lines at the master cylinder and then see what happens ?

[Quick Toy]

I went through a similar problem when I put in my Ford 9" with a rear disk setup. I had to do a bunch of math calculations to get the pedal force correct.
Because the cylinder volumes are different from the old drum hydraulic cylinders and the new disk caliper cylinders, the bore and stroke of the master cylinder needs to be matched to the new setup. Usually with the increase in the volume, now required with the rear calipers you need to go to a smaller bore master cylinder. (I know it sounds backward but remember you are dealing with area and pressure)
Sometimes you can find a similar master cylinder from a 4 wheel disk brake car, that may work with what you have, or you need to do the math and see what comes close swap wise.
(Remember that there usually are two pistons in the master cylinder. One is for the front brakes and the other is for the rear. The placement of the seals on the pistons will have an effect on the volume delivered. Hence, disk/ drum setups and disk/disk setups.)
(I ended up using a master cylinder off a 4 wheel disk 280 ZX. that had the correct bore and stroke for my setup. It was mated to my stock brake booster.)


FWIW
I had a hell of a time bleeding the brakes, thinking I had gotten all the air out of the lines, but the pedal still felt spongy. I got a tip from a old school guy, who told me to hold the caliper with the bleed screw facing up and lightly tap the caliper body with a hammer to free the bubbles that got stuck around the piston and bore...Worked like a charm, and a bunch of more air came out when I bled them again.


HTH
Paul

[Don't Panic]

I have a master cylinder kicking around that should work and I will try that. Not sure if I want to get involved with switching the lines around but it is an option if all else fails.

Regarding air in the line, I have been told that you can still pump up the brake to get a hard pedal. This doesn't sound right to me but does anyone know for sure?

In another forum, I had a guy telling me he had to bleed the brakes by pumping the fluid from the bleeder to the master cylinder using a syringe to get the air out. It is a wonder more people don't have brake problems if these methods are necessary.

[Don't Panic]

I just finished putting on a Mustang SVO master cylinder (four wheel disk brake car). It helped a bit, the car might be driveable now but it still doesn't seem good. With the car jacked up the rear tires will still spin when you lightly jab the throttle while standing on the brakes. The only difference being is that you can hit the throttle a bit more than before. When I changed the master cylinder I also removed the adjustable proportioning valve to make a direct line to the rear.

[Quick Toy]

Is the pedal still spongy, or just not holding the rear brakes enough?
If it is still spongy, sometimes a "bench bleed" of the master cylinder helps to get some of the air out.

What are the rear calipers off of?

I'm not sure about your setup but, have you adjusted the rear brakes, to take up most of the slack between the piston/pad and the rotor? My setup is self adjusting, and works with the "E" brake, but I preset it by adjusting the pistons first. (The heads screw in and out of the pistons)

Paul

[1 RARE T]

Question, do you have the rear jacked up off the floor and the front of the car on the floor?

Brake fluid does'nt like to travel up-hill.

How is the pressure when bleeding? Does the fluid trickle out or spray?

Mike

[Don't Panic]

The pedal isn't spongy, and the brakes don't get better no matter how much you pump the pedal. The rear calipers are the kind that self adjust with the emergency brake but the ebrake doesn't work. I havn't adjusted them so I probably should check them out. When I was testing the brakes the entire car was jacked up so there wouldn't have been a problem with incline. I don't think the fluid was comming out abnormally slow but I was working the brakes, I will have to check with my buddy.