1995 s10 blazer, 4.3 4wd automatic. Hit the brakes and the petal is hard with no braking of the vehicle - release and push the pedal again and the car brakes. Happens intermittenly with out warning. Sometimes you hit the brake and keep pressure (its a hard pedal) and eventually the pedal goes down and it brakes - but you never know!!

Recently replaced master and linings are ok.

Help is appreciated - its my wife's car so I hear about it over and over again!!

 

I can think of 3 possible problems, a faulty master cylinder, faulty vacuum booster or a vacuum booster check valve.
Pump the brakes a few times, then hold pressure on the brake pedal and start the engine. The pedal should drop a little as the booster adds assist. If the pedal doesn't drop and stays hard, it's in the booster , if it works normally and the pedal drops a little, it's probably the master cylinder (again).

 

Thanks for the info, I did the test and the results pointed to the master cyl again.

I checked on the check valve and the experts at autozone told me it was a filter, not a check valve. The dealer wanted $19 for it and the "HELP! parts" was about $6. So I will change that prior to making the big job.

Can you verify if it is indeed a check valve or only a filter?

BTW, I'm gonna change the booster also, just to shut the ole lady up and keep her offmy back! if you know what I mean!

 

OK OK, I figured it out, it is indeed a check valve - purchased one and did the blow test - my old one seemed to be sluggish so I sprayed it with wd 40 and cleaned it and it now works fine

Brakes seem to work more reliably now. But I noticed that when I depress the brake I hear a "squishing sound" from under the dash - any hints on what this can be??

 

Well guys, after cleaning the check valve (and yes it is indeed a check valve) the car has run well with the brakes according to my wife "not acting up, maybe you got it!".

But not to take any chances and have the wrath of my wife on my *** again, we're gonna change out the booster and master - the agony of her rathe is well offset by the $150 in parts.

Any clues on the powerboster installation? How hard is it to get under the dash and get to the 4 mounting nuts?

 

Umm...I could be alone here.

But, especially for a vehicle as old as yours, I would be concerned about maintenance induced failures. As a general rule, it's a good idea to go through troubleshooting processes and repair / replacements one step at a time. That way you know what fixed the problem, you minimize maintenance induced failures, and you make efficient use of your money and repair time.

If it's "not acting up", I would leave it alone for a bit. Keep and eye on it, and keep the $150 in your wallet for now. While avoidance of your wife's wrath may be worth $150, imagine what wrath would be associated with a potentialarray of problems created by tinkerin' with a bunch more stuff what don't need tinkerin' with!

Just my $0.02

Icarus

 

I am in agreement with dashyman. But I doubt I would go as far as doing the break booster. These don't normally fail till they hit the 20 yr marker. Somelast even longer then that. (looking over at my 74 that still has the original booster in it, but had had the master cylinder replaced 5 times. Then my 76 Chevy that is still all original and works like new still).

vacuum booster check valve and the Vacuum Supply Line are the two things I would worry about. Also the seal from the cylinder to the booster (another known issue with some setups). I have seen them with just a hair gap that was just enough to cause problems.