[RocktimusPryme]

Most of the aftermarket brake kits come with a bulky proportioning valve. However when master cylinder shopping I see master cylinders for Disc/Drum, Disc/Disc etc.

If you buy the master cylinder for the specific job, do you still need the prop valve?

[leeklm]

Worst case is you may need to install an adjustable prop valve for the rear. My lines are connected directly to the MC with no prop valve and all is good with my manual disc/disc setup.

Sent from my SM-G965U using Tapatalk

[66sprint6]

Prop valve, hold-off valve, whatever. You will need something to make the rears hold off for that millisecond so that they don't lock up early. The factory stuff is all over the place. I've seen the prop valves by the MC, on the frame, and also the short fat hold-off valves at the MC. It looks as though they've now standardized to the plate mounted prop valve with the little lines from the MC.. I know that when I get my disc brake conversion hydraulic lines, they will mate to that unit. That's all I want to do. I don't want to start making lines.

["QUICK-SILVER"]

MC for brake type and prop/combo valve to control pressure to the rear.

Rear drum MC's has or did have a residual valve where the line hooked up. That kept a slight amount of pressure on the rear line. I think to keep slack out. Not the same as stopping pressure.

Read just enough here on PY that I really don't know any more.
Clay

[hobbygto65]

There are a lot of cars out there that look like they have a proportioning valve but what there looking at is part of the brake warning system. They look a lot a like.

[ZeGermanHam]

Originally Posted by hobbygto65

There are a lot of cars out there that look like they have a proportioning valve but what there looking at is part of the brake warning system. They look a lot a like.

Isn't that because the prop valves frequently have an integrated brake warning sensor? That's how mine is anyway.

[RocktimusPryme]

I guess the over-riding thought was. If all you need is a master cylinder with the proper valving, why do the companies bother with the added cost of a prop valve.

[242177P]

Technically, it's a combination valve.

[Geoff]

Drum brakes need less pressure than disk brakes, hence the use of a proportioning valve to 'proportion' pressure to the rear brakes.

Another way of reducing pressure to the rear brakes would be use reduce the diameter of the pistons in the rear wheel cyls, eliminating the need, complexity & cost of a PV. I could never understand why Detroit didn't do this. Maybe the bean counters had a day off that day....
My GTO had a tendency to lock the rears, with stock drum brakes. I reduced rear wheel piston size & cured the problem.

A combination valve has the PV, plus one or more other valves such as metering valve, loss of system pressure valve, stop light sw valve.

[RocktimusPryme]

I know what a proportioning valve does. But then why are there different master cylinders for different brake layouts listed?

["QUICK-SILVER"]

Residual valve for rear drum doesn't regulate pressure out. That's prop/combo valves job.

Disc/disc doesn't use residual valve.

Residual valve in the hole where rear brake line hooks up is the only difference between disc/disc and disc/drum. The 1 1/8th" bore master cylinder on my old van (disc/drum) has the same casting number and bore size as disc/disc corvette.

Clay

[Keith Seymore]

Originally Posted by RocktimusPryme

I know what a proportioning valve does. But then why are there different master cylinders for different brake layouts listed?

Because each brake system is unique, depending on type (ie, power vs manual), disc vs drum, GVW, payload, wheelbase, center of gravity location, master cylinder bore, friction material, wheel cylinder dia, desired pedal feel, etc.

As an OE brake development engineer you would tune the combination valve (metering hold off portion, split point and slope of the front vs rear proportioning) for each general model type.

Buying a random proportioning valve/combo valve or master cylinder and slapping it on willy-nilly is like buying a random carburetor and throwing it on and expecting it to work. You might get lucky, but there are a million ways for you to not get lucky.

I'd start with the recommended M/C and put an adjustable valve on and tune it to what you want.

K

[Keith Seymore]

Plus - there are a million ways for a master cylinder to not work: bore diameter, disc vs drum (ie, reservoir size), reservoir size and location for hood clearance, port placement (front port goes to front vs front port goes to rear), port placement (inboard side vs outboard side), port size, port thread type (ISO vs English flare), cast iron vs aluminum/plastic, "Quick take up" vs non QTU, etc.

Lots of variables to comprehend...probably why the aftermarket has settled on just a few different types.

K