Bad brake booster check valve?

[Fett]

I am kind of at a loss on this one. It could be as simple as air in the lines, or a bad check valve...I am not quite sure.

I put on a new master cyl a few weeks ago, and used a force bleeder to flush all the fluid and remove any air. I have no leaks. This is stock brakes all around. Since the new master, flush, and bleeding...my pedal has play for the first inch or so. The brakes grab fine, but the pedal goes down a little more than an inch before it gets stiff and the brakes start to grab.

If I pump the brakes, it doesn't get any better. I can pump all day long, but the pedal never gets firm. But if I put a quick load on the engine the pedal gets stiff. In other words, if I am in gear and rev up to 2500rpms, then quickly let off the gas and hit the brake, the pedal is stiff. If I am taking the car up in rpms on the highway the pedal feels stiff. But if I rev the car out of gear, it doesn't do the trick. So it seems that only with a load on the engine does the pedal get stiff, and only when coming down from higher rpms.

Just seeing if anybody had any thoughts.

 

[Fett]

I forgot to mention, this is not really a new thing. It was happening prior to the new master....the deal with the soft pedal. But before the master swap (the last one was leaking), the pedal would NEVER get stiff. I am pretty sure there was air in the system back then. Which is why I don't think it is a problem with air in the lines now.

 

[89streetbeast]

might check and see if your rear brakes need tightening. ive ran into that a few times where the pedal was a little softer than it was before i did a brake job. i usually just adjust them to where theres a little drag with the drum. doesnt help much if you like to do burnouts tho lol.

 

[cobradvm]

Did you adjust the rod from the booster so it was the correct length for the new MC? Sounds like it might be a bit short, if there is play for the first inch or so. To check the check valve, start the car and let it warm up. Once it is warmed up switch it off, wait a few minutes and pull the check valve out of the booster. You should hear a rush of air into the booster if the check valve is working ok.
A bad booster will be like manual brakes - you have to push real hard to get it too work. FWIW I had a bad brake booster, but it held a vacuum fine. Once I swapped that out, the brakes were great - this was on a cobra 4 disk upgrade.

HTH