Hmm. No one has mentioned the residual pressure check valves in the M/C. I forget now whether it is the discs or the drums that use them, I want to say that they are used with the drum brakes and the discs do not use them.
Glad you brought it up! Essentially correct. At least "officially" a drum brake setup uses a 10psi rpv, while a disc brake setup uses a 2psi rpv.
But that's only if you choose to use them, or if you actually need them.
They had two basic important functions back in the day.
1. Was to keep fluid pressure against the pistons in the wheel cylinders, to better keep the seals from leaking and from letting the pistons back off even further than the shoe anchor pins would normally allow.
2. Was to keep brake fluid from migrating "down" from the wheels to the older, VERY LOW mounted master cylinders. Back when you checked and filled them by a panel in the floorboards!
Hasn't been a thing for over 60 years though, I'd venture to guess.
Most modern replacement masters (and probably all aftermarket performance types?) don't even have them as part of the master anymore. Not needed with high-mounted masters which are the norm, and not needed with most wheel cylinders coming with internal springs to do the same thing.
But it pays to know if you have springs or not, as it might effect how you build the rest of the system.
When we do a disc brake swap on an older master though, where they were more often present, we remove the one from the disc brake circuit.
Drums have springs bringing the shoes back off of the drums. If the shoes are out of adjustment and there are no check valves in the M/C you will get low pedal, as you have to use u pedal travel just getting the shoes back out to the drums.
True, but only for a short time. Eventually, if not immediately, the pressure of the springs is WAY more than enough to overcome any residual pressure check valve. So your drum brakes (assuming they're working correctly and are of the design we're all more familiar with) will ALWAYS return to their anchor points. So adjustment is critical whether you have a valve or not.
So yes, the low pedal can definitely come from the drums if they're out of adjustment. Unfortunately, at least in my experience, the auto-adjuster mechanisms are rarely super efficient. Even when they're working, they don't always keep up with lining wear.
Discs don't use any springs when the brakes are released, they depend upon a tiny wobble in the discs that push the pistons ever so slightly back.
And some (maybe all of them nowadays?) rely on the design of the caliper's piston seal to just barely apply some inward/backward pressure to act as a pull-off mechanism. So even without that slight wobble, most disc pads will be pulled ever so slightly off to keep from dragging.
Are they installed in your M/C and which ports have them? Somewhere in this thread you mentioned a 3/16 to 1/4" gap between the pads and the rotors? That is entirely wrong.
I didn't catch that. But yes, that's entirely too much gap for just about any part of a brake system.
Side bar, My U14 has discs up front and drums in back and NO booster. works great. Great modulation and if really needed I can lock them up.
Same here with my '71. No booster, and original (actually the first replacement) master cylinder. Replaced the master in about '81 or so maybe? It definitely had the rpv's in the ports. You could feel them with a poker thingy against the rubber and spring pressure.
I purposely drove it around the first week of commuting with the RPV still installed just to see if it would lock up the brakes. It did not, and drove just fine. But I still took it out because it was the correct thing to do and I didn't want to have any trouble on a long trip where the brakes might have heated up more.
Paul
