Originally Posted by cimplexsound
My brakes started acting up on my Tioga recently. My mechanic and I have been through thick and thin trying to figure out what could possibly be wrong. After I drive it about a mile the brakes will freeze up and I can't go anywhere. First symptom I can't get it past 25 MPH, then I have a hard pedal, them I'm stuck on the side of the road with frozen brakes. We're already $600.00 into repairs. We both think the master cylinder is shot. But he is checking the brake booster just to make sure that is not a problem too. My mechanic had to come out and rescue me twice yesterday. By loosening the mounting screws on the master cylinder to relive pressure on the system we were able to get it moving again long enough to get it back to the shop for repair. I never had a problem last winter. This big beast got me to Arizona and back without any trouble. And she runs great. It's just the brake problems. Napa Auto has the parts if that is indeed the problem. I am anxious to get out of here before the snow flys. So what do you think?? Everything else is in perfect working order. What could be wrong???
As has been stated, you don't say what make/model/engine/year/type of coach you have so, some of that info might make a difference in what kind of answers you may receive. Second, from your description of a temp remedy of the problem, you (and your mechanic) loosen the nuts from the brake power brake booster to the master cylinder which, apparently allows for the piston in the master cylinder to RETURN to the AT REST POSITION, which, allows for the fluid in the system to relieve its own pressure by re-entering the storage chambers in the master cylinder.
You see, in a drum brake operation, hydraulic pressure in each wheel cylinder is applied to the shoes, to push them outward, towards the drum, to apply brakes. Then, when the foot is removed from the pedal, there are some fairly strong "return springs" that will pull the brake shoes, back to their home position and, along with that, the fluid used will travel back to the master cylinder and, all is well since the shoes are no longer against the drums.
In a disc brake operation, there are no RETURN springs. Only atmospheric pressure is existent and no pressure in the system is there, when the pedal is released due to the large spring in the power brake unit, that allows for the piston to do a full return.
Now, to me, that's kind-of pointing at the power brake booster. In a power brake booster, vacuum used to assist the pedal in applying pressure to the piston in the master cylinder. But, there is also a large return spring inside that chamber that pushes the diaphragm back towards the driver, so that the piston in the master cylinder can return to the "at rest position" when the pedal is released. And, when that happens, like stated, that will open up the ports in the master cylinder to allow for the brake fluid that was used for pressure, to return to the holding chambers.
And that means no residual pressure is left in the system. Well, it appears that you're system IS HOLDING PRESSURE! And, that when you back off the retaining nuts that retain the master cylinder to the booster, the piston is allowed to self return and, your pressure is now relieved and, your brakes are not held in the "on" position.
So, with all that being said, if you've done this "back off the nuts thing" a few times and it cures the problem, EVERY TIME, then I'd take a look at that brake booster and see what, if anything can be wrong, with it's internal free travel, that would limit the master cylinder to return to it's home position. Just a thought here. Let us know what you find out.
As a side note, while my theory might be valid, it could be just the master cylinder causing the issue. But, if it was, then releasing the retaining nuts that retain the master cylinder to the booster, should have no effect. It would still be holding pressure. Again, just some thoughts