Residual Pressure valve

I believe when you change out rear drums to discs you have to add a residual pressure valve.

My front disc brakes are Ghia, and I think the rear disc brakes are a CB kit. It has the emergency brake built into the rear caliper.

If I have a brake master cylinder specifically for disc brakes - front and rear - does that master cylinder contain some sort of built-in residual pressure valve?

i think you are talking about a proportioning valve.

I'm getting an adjustable to give my rear a bit more bite.

David, yes, the residual pressure valve is needed when converting to rear disk brakes. If you don't have it you will discover you need to pump the brakes a couple of times to "bring up" the rears, and that is not good if you need to stop in a hurry. And no, there are no residual pressure valves built into a VW master cylinder IME. I added the residual valve and was much happier with my stopping...as a matter of fact I ultimately added 3 of them, one to each side of the front brakes and one for the rears. Instant hard brakes first time every time. I am not certain the fronts are necessary but the brakes do work well now....I just added them to my "new" Speedster I am currently working on also....

Residual valve(s) is correct - it keeps pressure in brake system (fluid draining back to master cylinder). Seems too much pressure on disc brakes would cause the pads to drag and overheat. Proportioning valve could come in play too it you want increased braking done by rear brakes vs front one. I'm not sure if below article is correct in that new MC don't have them at all (one article said VW dropped them in '68)- it shows a picture of what the OEM ones looked like. Looks like there are metering valves too - for those with rear drums (which don't seem to contribute much to braking).

http://www.shining-wit.net/tina/buggy/design/brakes/index.html#Residual%20Pressure%20Valves

As mentioned, a residual pressure valve and a proportoning valve are totally different. A residual pressure valve (usually 10 lb and 3 lb are availalbe. A 10 pound unit us """usually"" used if you have rear drums and front disc's. Less volume or pressure is needed to activate the front disc's than the rear shoes. In other words, the shoes have to travel farther to engage than the front pad movement to the discs. The 10 pounder is installed as close to the master cylinder as possible and keeps 10 pounds of pressure in the lines to the rear shoes.

A 3 pounder is sometimes used on disc brakes. Again, this keeps 3 pounds of pressure in the lines to the disc brakes.

Since 80% of your braking is done by the front brakes, a proportoning valve is installed, usually between the mastercylinder and the rear brakes. Weight shift is the reason for having this valve because weight is shifted forward during braking thus making the rear of the car lighter and prone to early brake lock up.

This valve limits the amount of pressure applied to the rear brakes otherwise, they will tend to lock up when applying pressure to the brake pedal. By adjusting this valve, you can limit the amount of pressure to the rear pads so they don't lock up before the front brakes lock up.

Well that's another thing on the to do list. I have exactly the same set up on the Ghia from MC to front discs and new drums in the rear. The Ghia stops on a dime and the Speedy has low pressure ALL THE TIME!

I'm going to convert to a dual MC and dual reservoir in the spring and will look into proportional pressure.

Larry:

Anybody's proportional valve better than others for a VW?

I just used a 3 lb. residual to the rears as CB recommended. Works fine.

~WB

I got the CB performance rear cals. with parking brakes and there was nothing in the kit about the valves. Where does one acquire such devices and how does one know which calibration is desires. I saw a suggested 3# one mentioned here. o, where?

Thanx,

GG

Summit Racing has the Wilwood 2# and 10# ones -

http://www.summitracing.com/search/Brand/Wilwood-Disc-Brakes/Product-Line/Wilwood-Residual-Pressure-Valves/?autoview=sku

My guess is they will be SAE (you need the 2#) and you need metric - so may have to cut brake line and use SAE fitting inline.

http://www.cbperformance.com/catalog.asp?ProductID=1130

Gordon, Sorry for the delay, I'm at the sand dunes driving around in my sandrail and I have limited internet access.

I usually purchase the Wilwood residual pressure valves... Although I can't remember the brand name, I believe I've also used proportioning valves marketed by Wilwood.

George - I have the CB kit with rear discs also. I have no obvious residual pressure valves. Hence the reason for my original question above. Intellectual curiosity on how things work. My system has always worked fine.

JPS told me to "exercise the hand break frequently" back in 2001. Never had an issue. The hand brake is actually a lock, built into the rear calipers, actuated by the emergency brake cable. It is internal to the caliper and appears to be hydraulic, but I suppose it could act mechanically onto the caliper piston?

I have used residual pressure valves in applications where the MC is at a lower level than the caliper - M/C under the floor in a hot rod for example. The idea is to prevent fluid from draining back into the MC

Is this really needed on a Speedster, using dropped spindles on the front. I am doing discs all round this right now without residual valves and am about to find out. In any case I do not have the valves, and they were now supplied in the "kits".

Will use the proportioning valve on the rears from the get go as I have one and think it will be a good addition.