: Low brake pedal.



Wheelman322
08-07-07, 02:40 AM
The brake pedal in my 78 FWB has always been low since I bought it. In the past couple of months I replaced the Master cylinder, all the pads and rotors (front and rear), and all of the calipers. The system has been bled as well and the pedal is still not as high or tight as the one in my 80 FWB which is rolling on all original hardware. Could it be the power brake booster?

Mud
08-07-07, 09:04 AM
Was the master cylinder bench bled first before installing? That is the first start to a tight system. Also, double check that you got the right MC, mistakes can happen.

The booster only provides vacuum assist to push on the master cylinder piston. If anything, a bad booster would make for a harder pedal. However, there is a little rod that is between the booster and the master cylinder - that's what actually pushes the piston. If the piston position in the new master cylinder is a little further in from the end, or the cylinder is for a different car/booster (remember that many replacement MC's can be used for a variety of applications) then that little rod has to push in further to work the MC. That results in the pedal moving down further.

What it really sounds like is that you may still have air in the system. Aside from manual bleeding with a helper to pump the brake pedal, try to simply gravity bleed the system. Take the cap off the MC, loosed the bleeder at each wheel and just let it drip. Somewhat messy and slow, but effective. Also, not trying to doubt your ability, but you did install the calipers with the bleeders at the top, right? Sometimes they can be installed reversed (wrong sides) with the bleeders positioned lower and they will never bleed out properly. As you are bleeding, tap the caliper with a little hammer just in case there is an air bubble trapped somewhere.

Some cars are just a bit more finicky on the brake/bleeding process, don't know why. When I replaced the calipers on my 86 FWB, I was not happy with the pedal. I had used a mityvac to bleed out the system, so I went ahead and did the gravity bleed. Both calipers had a bit of air trapped, problem solved.

Oh, one other thing, old brake hoses can certainly contribute to the lower pedal. If these are old or God forbid the original hoses, they should all be replaced. Don't take a chance with bursting a hose.

The Ape Man
08-07-07, 09:28 AM
I noticed the same thing years ago when I first got my paws on a 1981 model.

Try to rule out mechanical problems first as outlined by MUD.

I don't remember if '78 was a problem but I know for sure that '79 RWD Cadillacs came from the factory with poor brake pedal feel. This was caused by additional amounts of brake fluid needed to apply the front calpiers. Older front calipers didn't retract as easily as the later ones which were designed with fuel economy in mind. When you first hit the pedal, the front calipers need extra fluid before any braking happens. Rear drum brake adjustment also effects pedal feel so make sure it's right.
Later master cylinders allow a larger displacement of fluid initially with the same pedal travel. The booster isn't your problem but they were known to fail before those cars were used up. If yours gives up then maybe consider converting to the later setup. You would probably have to swap the proportioning valve also.