[kcabpilot]

This is not something that snuck up on me, it just one day started doing it.

What happens is that the brakes work normal until slowed and almost to a stop at which point the pedal feels rock hard as if there is no boost and I have to really stomp on it to get the car to fully stop. There is no indication of any sort of intake leak, no loss of brake fluid, no pulling left or right. I tried clamping off the booster line for comparison and it's obvious that the booster is working because it doesn't feel anything like it does when I disable it.

Panic stops are normal - of course in that situation I am already stomping hard on the pedal, I get no lock up front or rear as would be expected if the proportioning valve were malfunctioning.

It is the same even if I step on the clutch and let the engine idle prior to applying the brakes so it's not a matter of insufficient boost vacuum but it really feels as though boost has been lost, that's the best way I can describe it. Pumping the brake pedal has no effect.

Any ideas?

Edit: I should also add that it does this right from the get go, it's not a heat fade issue. Pedal travel is normal although when it goes hard there seems to be very little modulation or movement beyond that point.

 

[bianchi2]

.....hum.. i am running my brakes without power brakes.. even with 4 piston claipers it's not hard to stop the car....

 

[papajam]

Check/clean the vacuum check valve.

 

[kcabpilot]

Thanks Jim

I pulled the check valve and flushed it this morning, also did a vacuum leak test on the booster. It leaks down very slowly, easily kept up with using small hand pump. I imagine that's probably normal as there must be a bleed function inside the booster to equalize pressure when the engine is off (I would think) It's still not acting right.

In addition to that I did a regular booster check by pumping the pedal then starting the engine with foot resting on it to see if it goes down - it did.

So, despite how it feels I'm pretty sure the booster is working okay and I'm moving on to the hydraulics. It's time to do a complete flush anyway.

 

[papajam]

...as there must be a bleed function inside the booster to equalize pressure when the engine is off...

Negative. There's no bleed function. With everything functioning as it should, the booster should hold vacuum indefinitely.

 

[kcabpilot]

Okay then, back to the booster.

I wanted to make sure it wasn't my test setup that was leaking down but after rummaging around in the tool box I was able to Tee a vacuum gauge into the booster line. It pulls about 20 inches at idle but bleeds down to zero if I step on the brake. If I shut the engine off it holds pretty good with a very slight leak down (could be my setup) but if I tap the brake it goes flat to zero.

So it looks like my booster is toast, what I'm seeing on the gauge would explain perfectly what I am feeling in the pedal. Now I need to source a replacement.

Thanks again for the help Jim

 

[kcabpilot]

YIKES! - $340 for a new one.

 

[kcabpilot]

Update:

I had Larry at Alfa Parts Exchange send me a salvaged booster for $100. The new ones are $380 and honestly, I have never in my life had a brake booster go bad. Well, up until now that is. In fact, I don't even know anyone who has. But mine was definitely bad, swapped the replacement in today and everything is back to normal.

It wasn't as bad as I had feared. There are four studs that hold the entire pedal box to the firewall, two in the engine compartment on top and the two, easy to get to lower ones on the inside. Then you can separate the booster from the mounting box.

Of course, chances are a thousand to one that you'll NEVER have to do this.

 

[papajam]

Happy endings are a good thing. Congrats!

 

[60sRacer]

The vacuum hose from the check valve could also be leaking. DId you check that?

Robert