Dump truck. Chassis mount booster. Replaced 5 years ago with rebuilt (Cardid, I think, from NAPA). When I replaced it I also replaced vacuum lines and vacuum check valve (the original plastic check valve was junk-would sometimes stick open, blocking intake suction from reaching booster-with new valve this cannot happen). Current problem: intermittent hard pedal (no boost), yet vacuum gauge reads correct (about 20+). Holds vacuum for days with engine not running, so obviously no vacuum leaks. Yesterday, at low speed, I got an intermittent "chatter" when I depressed the brake pedal, and it felt like the brake shoes themselves were grabbing intermittently. I am about to order a new (rebuilt) unit, as I cannot think of anything else that would cause this. Any ideas? PS: Brakes themselves need adjustment, but I don't see how this could be related.
It can, I had these exact symptoms on a '77 IH. My drums were too worn too get the adjustment up to snuff. I don't know how it causes an intermittent problem, but, I know it did. What do you have to lose, except some knuckle skin, by adjusting them before buying parts to see if it fixes it?
Would it cause a hard pedal and no boost from the booster??
Yes, because the stroke of the booster is limited, and if your wheel cylinders need more oil than that, you are relying on only what the master cylinder can push.
I had thought that the booster was like the master cylinder and, by pumping, could push as far as it had to. What you say makes sense, especially since the brakes have been getting pretty low. I'm going to have them adjusted and/or replaced. I tried to turn the bottom adjusting wheel and could not budge it, and I doubt I would be able to access the top one. Thanks!