buddy of mine has a 71 cutlass with a 350 in it with power brakes. has disc brakes in the front and drum in the back. we took it out the other day and the brakes seemed ok but about 40 minutes into the drive the brakes became very very hard to press. any ideas what this is or what caused it.

thanks in advance

 

Did the car still stop if you pressed hard on the pedal?

Sounds like a loss of power assist. That would be caused by a loss of vacuum. I'd be checking the vacuum hose to the master cylinder.

 

To be technically correct, the vacuum hose runs to the power booster, not the master cylinder.

 

thanks will pass this on to him and see if it fixes the problem

 

Less common is the diaphragm inside the power booster failing (cracking, punctured, etc). If you have good vacuum to the booster and no leaks in the hose going to it, check the atmospheric port on the booster and see if it is continuously drawing in air. Sometimes if this is the case you can hold down the brake pedal while parked and listen to the booster, you will hear a steady hissing. All of this while the engine is running...

 

I had exactly this problem with my 72 last year. It was a combination of 2 things.
1. The master cylinder was shot.
2. The disc brake piston was partly seized.

Both of those are easy to fix, but make sure you don't get brake fluid all over the paint - it's corrosive as heck. When you're done you'll need to help him bleed the system. Should be good as new.
Oh, a caliper rebuild kit from NAPA is around 8.00 per side vs 40.00 for a new caliper. Note: be very careful with the caliper bleeder screw. If you bust it (good chance if the car hasn't been serviced for a long time) you'll be buying a new caliper.

If the power booster is still creating good vacuum (easy to test - just pull the hose when the engine is running) leave it alone. Agree with AZ455 on this.

The rebuild I did on mine makes it feel smooth and just like new!

 

I had a caliper seize on my Olds also, after a brake line change. It seized damn near fully applied though. I felt pull to the pass side on my test drive, when I came to the stop sign at the end of my street and smelled my brake pads burning up I knew. The 455 hardly noticed. I was going to rebuild, but I got a reman GM caliper for $13.

 

Note: should mention the 'low cost' of the calliper you're stating should be 'after core', right?

 

Ah yes, forgot to mention that. $8 core.

 

ok so my buddy and i checked the hose from the carb to the power booster and all seems fine and "looking" at the the power booster and master cylinder they both look new

my buddy just bought this car and most of the car looks rebuilt, replaced etc

is it possible the booster is bad??

 

is there a way to test the booster??

 

Read my first post above.

https://classicoldsmobile.com/forums...tml#post460923

A few other tests besides what I mentioned above:

Engine off: pump the pedal several times until it is very firm. Depress the pedal as far and you can and hold it. Start the engine while holding the brake pedal down, as soon as the engine starts the pedal should drop some. This is normal operation.

&

Release the brake (engine running) and then turn off the engine. Pump the pedal several times slowly, pedal travel should decrease gradually over several pump of the pedal. This is normal operation.

&

While the engine is running depress the brake pedal firmly and hold it. Shut off the engine and wait 30 seconds, pedal still depressed. The height of the pedal should not change during this time. This is normal. If the pedal raises slightly over the 30 seconds, this is not normal operation and there is an internal leakage.

 

 

I'm assuming you have checked the fluid, if only the rear reservoir is full you could be stopping on the rears only( that is if your year car has a split master cylinder) .I'm getting way out of my years of expertise here...Tedd

 

FINALLY!!!!!!

my buddy and i were finally able to put a new brake booster on the car and she runs and stops great

thanks for everyones help

 

Good job. From your earlier posts I thought the MC and BB were new so I didn't suggest anything about changing either of them. Should have picked up on that when you responded to AZ455 about hard pedal. Glad it's fixed and running great.