Going to 4-wheel disc brakes. Change master cylinder instead of proportioning valve?

[V6UnderPressure]

I've read a few threads on other boards about some individuals using an F-body master cylinder when converting to rear discs. I figured this would be a simple way instead of having to replumb a little bit. Has anyone done this? And if so, what years would work?

 

[Slow91z]

That doesn't make much sense to me...the proportioning valve will still be dropping the rear pressure to drum brake levels.

 

[V6UnderPressure]

I wondered the same thing. I didn't see how it would work but some claim it did???

 

[cruzn57]

the proportioning valve does NOT reduce pressure, it delays pressure until the fronts have reached the prescribed pressure ( valve has that psi # built in)
you can use it with 4 wheel disc, it will function fine.

 

[HelloBozos]

You need to modifie the proportioning valve if want to use factory one ,there's a thread about it already

If you don't you'll have to pump your bakes once before using.....

 

[charlief1]

What most don't understand is that a drum brake operates off volume of fluid more than pressure and discs operate more on pressure than volume. This is because the shoes and wheel cylinder move more than pads and piston. The prop valve is more like a residual valve to hold some pressure in the drum brake lines because they move more and have spring pressure pulling them inwards where the disc brakes float with lower clearances between the pad and the rotor.

Go ahead and gut the prop valve or get one for front and rear discs and then look for a master off an 84 or so Riv. They had 4 wheel disc on them and the bore will be smaller to help increase the pressure to the discs.

 

[coach]

What most don't understand is that a drum brake operates off volume of fluid more than pressure and discs operate more on pressure than volume. This is because the shoes and wheel cylinder move more than pads and piston. The prop valve is more like a residual valve to hold some pressure in the drum brake lines because they move more and have spring pressure pulling them inwards where the disc brakes float with lower clearances between the pad and the rotor.

Go ahead and gut the prop valve or get one for front and rear discs and then look for a master off an 84 or so Riv. They had 4 wheel disc on them and the bore will be smaller to help increase the pressure to the discs.

WOW...What would we do without you Charlie??? I was going to ask a similar question about needing to run a different line size from the Powermaster if I went discs in the rear. Do you know what to do if your using the PM and rear discs? Let me know buddy.

 

[salvageV6]

You will still need one of these out back to lower the back bias.

Powermaster or vacuum.

http://www.amazon.com/gp/product/images/B001O078VO/ref=dp_image_0?ie=UTF8&n=15684181&s=automotive

 

[charlief1]

WOW...What would we do without you Charlie??? I was going to ask a similar question about needing to run a different line size from the Powermaster if I went discs in the rear. Do you know what to do if your using the PM and rear discs? Let me know buddy.

With the powerbastard you can reduce the line size to the same as the fronts Donnie. That should keep the pressures where you want them. One other thing you can try is to put a 2 lb residual valve inline rather than the stock 5 lb prop valve set up.

You will still need one of these out back to lower the back bias.

Powermaster or vacuum.

http://www.amazon.com/gp/product/images/B001O078VO/ref=dp_image_0?ie=UTF8&n=15684181&s=automotive

Not if you do the research and get the right parts. After all, GM, ferd, and chistler didn't need an adjustable valve when it came to 4 wheel disc brakes.

 

[scot w.]

You have to remember that GM used many different combination valves on these cars so who knows what one you have.... Some had a removable cap for the rear valve to modify and some Did Not..

On my personal car we installed my BAER brakes & Proportioning valve and never touched the factory combination valve during or after the up grade and my brakes work Fantastic stopping my car from 75mph in 2 seconds or under!! This was filmed & timed at the 2011 RTTH "BAER brake Speed/Stop Challenge"..

I spoke to Rick @ BAER and informed him of this post... Hopefully he will chime in to further the Prop valve info..

.

 

[WHITE T]

What we always do is match the master cylinder to the bore of the front caliper. The rear caliper piston size should be roughly 50% of the front then can be adjusted accordingly with a proportioning valve.

If you are just changing to disc in the rear an adjustable valve is all that should be needed. Just be careful with some of the rear disc conversions, as they use really large piston area calipers and hydraulically everything is mismatched. (rear wants to lock before the front, and a prop valve does not help as it is a volume problem, not a pressure problem).

The factory combination valves in most case (talking about all cars) have 3 functions. 1) They distribute the hardlines throughout the vehicle, 2) in many cases there is a low pressure warning light that will put a light on the dash, 3) built in residual valve to hold pressure on the drum brake so the shoe doesn't fall far from the drum (this is on cars that were equipped with drums).

In cases where we can modify the rear portion (follow hardlines to it and open the valve) we will gut it (take everything out other than the seal so it doesn't leak) so fluid runs straight through then all adjusting is done with the adjustable prop. valve. In some cases these valves can not be modified, and we have had customers leave them in place with success.

The reason that factory applications don't use prop valves is because the brake is engineered as a whole. The front and rear caliper piston area is selected and taken into account with suspension, tire width/height, wheel size, weight distribution etc. Once things are modified (rear calipers are used in applications they were not intended for) a prop valve is always recommended and a useful tuning tool.