An magazine article says (some or all?) drum brake MC’s have a one way valve in their output port so to maintain some hydraulic pressure in the brake line even after the brake pedal is released. Said to keep the wheel cylinder seals slightly pressurized to prevent leaking at the wheel.
Is this correct? I’m not aware of any such MC valve for my Ford 4-wheel drum braked truck.
Never heard of that. Can’t be a simple one way valve, almost all the pressure has to leak off.
I’ve never seen anything looking like a one-way valve in any drum-equipped MC I’ve replaced or rebuilt. I suppose it is possible it’s just that the MC’s piston & a o-ring seal is positioned slightly ahead of the output port hole. So when you press on the pedal it forces the fluid out towards the wheel freely, but when you release the brake pedal , the fluid is drawn back towards the MC, but then the output port-hole is closed off just before the piston retracts fully.
I don’t understand how keeping a slight pressure in the line helps to seal the wheel cylinders though. It seems like any pressure will cause a leaking wheel cylinder to only leak more.
I’ll pass. Don’t know, don’t care. My last drum brakes was 40 years ago where my 86 park ave had rear drums and front discs.
Yes many of the old drum brake system had a residual check valve in the master cylinder to keep a little pressure on the wheel cylinders incase they seep fluid to keep air from entering the brake system…
IIRC, the old frame mounted master cylinders did this as well as the newer, but still older firewall mounted MC’s…
And here’s some discussion about how they can accidentally be installed in disc brake master cylinders: