I’m trying to help a friend out with the brakes on a 56 Nomad. Originally a treadle vac car Treadle vac gone, typical vacumn booster on firewall, disc/drum master GM metric calipers New flax hoses Small rotors (fit 14” chrome reverse rims) Combination valve provided as part of entire kit above from ABS Car drove and worked fine for about 12 years. Then the right front caliper overheated, and was dragging hard, got pretty hot. Rotor blue, bearings toast, grease melted down in a puddle. So, new parts, both sides, rotors, calipers, bearings, hoses. Brakes won’t bleed, almost no fluid out the front ports. ABS says combo valve failed, they don’t use that one any longer, and it cannot be reset. New combo, bleed brakes, done. Short trip around the block, feels fine. First longer trip, 5 miles or so, front brakes starts dragging again. Today we pulled the master cylinder off the booster. Brake pedal under dash has clearance. Cylinder on booster (cylinder style, not a rod like I’m used to) moves out when peddle pushed and engine off, and back when released. He starts the car while I’m watching and the instant it starts and has vacuum the cylinder moves out as if to apply the brakes. Measure cylinder to master cylinder, it’s within I/8” at rest. So when the car starts, it moves enough to possibly apply the brakes. Spin front wheel, light drag with engine running, turns free when engine off and I pulled the check valve releasing the vacuum on the booster. Being a Ford guy, I’ve never seen that style booster. No adjustment in it. Anyone ever seen a booster fail this manner? Right now I’m beginning to think the original problem was not a stuck caliper, but the booster lightly applying the brakes and not releasing the pressure. Any suggestions welcome.
No drag with the booster disconnected, but no boost either obviously. Back of my mind is even though we bled it with the engine off, clear new fluid at all wheels, is there still a minor preload on the master cylinder not releasing enough to work right. I don’t have all that clear an understanding of the inner working of a master cylinder compared to say a master for a clutch.
You need to carefully measure the distance between the booster output rod and master cylinder primary piston. With full vacuum applied to the booster (about 20" engine or external might vac) there should be some clearance, maybe .015"-.020". IF there is no clearance or adjustment available, the booster and/or master cylinder are not correct. And, no, don't shim the master to solve the problem!
Bob, you’re confirming my suspicions. With full vacuum not only is there no clearance , there’s probably 5/16 to 3/8” preload on the master cylinder. As I stated, at the point you can feel the brakes drag spinning the tires by hand. We did shim the master, and that solved the drag problem. But it left a dangerously low pedal and very weak brakes. Didn’t leave the property for that test. I’ll rustle up a new booster. What’s bothering me is the years it worked before, and that I’ve never heard of a vacuum booster failing by putting the brakes on for you. A ghost?
It sounds like the check valve on the pedal rod may have failed meaning that at rest you have vacuum one side of the booster diaphragm and atmospheric pressure on the other. It’s an unusual failure but would explain your symptoms
Owner talked to ABS this morning. They stated it’s a dual diaphragm and if one fails piston will move out and apply the brakes. Same result as what you’re describing. I don’t know enough about how they operate to know. At any rate, they said if he’d ship it back they’d do a 1 day turnaround on a rebuild. It’ll be on the bench tonight and at UPS in the morning. I’ll report back. But if I’d known such a thing was possible, I wouldn’t have been so quick to diagnose it as a stuck caliper and kept looking. Damage was done anyway, but would have had him back on the road quicker. Thanks Oneball.
The Jalopy Journal
