larryseed

I have a hydrovac to use. Just change to disc on front. Would like to use the same m/c. I thank there is 10 psi residual at the m/c don't know for sure. Do I use the hydrovac for front disc and rear drums with 2psi residual valve front and 10 psi on rear. I know I have to use a proportioning valve.
Thank Larry

wmf62

remember to take the check valve out of your master cylinder or your front brakes will drag.
Bill

Plasticman

While I won't say you "don't have to use a proportioning valve", there are several guys here on the forum that are not using one (including myself on my 62 with front disc brakes).

From our experience, we removed the residual pressure valve (check valve) completely, so we have no residual pressure to either the front discs or rear drums. Any residual pressure to disc brakes will cause them to constantly drag. And we do not see any side affects in not having a residual pressure valve to the rear drum brakes (no issues with one end "braking" before the other - which is the reason for adding the residual pressure valve). Just make sure your drum brakes are properly adjusted.

Also, make sure your single master cylinder is in "good" shape (you have to disassemble it to take out the check valve anyway, so purchase a rebuild kit at your local NAPA store). And don't forget to replace your rear axle brake flex line (to confirm the new system is "good to go").

Plasticman

1955 copper

Also, make sure your single master cylinder is in "good" shape (you have to disassemble it to take out the check valve anyway,

What valve in the original MC , what's it look like?

wmf62

at the very front end of the m/c bore, between the end of the bore and the end of the spring, is a check valve that kind of looks like a 'pith helmet'. remove it.... sometimes it comes out attached to the end of the spring when you pull the piston & spring out...
Bill

1955 copper

I added a booster in my 55 Corvette having drum brakes years ago and they work great and dint remove that valve,???

Plasticman

[QUOTE=1955 copper;1559669046]I added a booster in my 55 Corvette having drum brakes years ago and they work great and dint remove that valve,???

If you still have drum brakes, then leave the valve in. It only needs to be removed if you convert to disc brakes.

Plasticman

larryseed

Ok I got it now. But do I need 2psi on front disc and 10 psi on rear durms? thanks for the help. I guess I am getting old. Use to build drag boats> No M/c on them. Larry

Plasticman

I am not running any valves, and see no side affects (with front discs and rear drums). WMF62 has discs at all 4 corners, and has no valves either.

Adding any check valve (that holds pressure) in a disc setup will add to brake drag. Normally a 2 PSI check valve is only used on disc systems IF the m/c is at or below the level of the calipers. That is not the case in Vettes.

I know what the books and experts say about using the valves, but our practical experience says they are not needed. Can we run into a situation where they are needed? Possibly, but not so far.

Please see the below info that is direct from ECI's web site. It shows the functions of each item. Note that ECI makes aftermarket brake systems, including for C1 Vettes.

Hold-Off / Metering Valves are used in the front (disc) system of a disc/drum brake system. They provide a "hold off" function to allow the rear (drum) brakes to actuate first. This function is very important in making the system function in the correct sequence in a rear wheel drive car. The rear brakes are always actuated first. This function is built into most factory type disc/drum combination valves. Make sure you have a metering/hold-off valve in the system either as a stand alone valve or as part of a factory valve.

Proportioning Valves go in the rear brake system (disc or drum) and provide for control of the rate of pressure rise to the rear brakes -- just the rate at which it builds up. Sooner or later the rear brakes see full master cylinder discharge pressure. The purpose of this rate of pressure rise control is to compensate for the reduction of weight on the rear wheels due to forward weight transfer during braking. In short, it eases the application of the rear brakes to help prevent rear wheel lockup. Factory combination valves have these built in -- make sure you know what you're getting, too little rate of rise is as bad as too much. You may not need one of these valves depending on the compatibility of your vehicle's brake system components.

Adjustable Proportioning Valves allow for fine tuning of the rate of pressure rise to the rear brakes if you have a lock up problem. If you have a lockup problem, experiment with the setting of the valve to eliminate lock up for all but all out panic stops. Good luck!

Residual Pressure Valves are used in a both front and rear brake system as follows:

2 PSI Valves - These valves are used in a disc brake system only and are required when the master cylinder is at, or below, the height of the calipers. It's purpose is to act as an anti-siphon valve preventing the brake fluid from siphoning back into the master cylinder when the brake pedal is released. Even if the master cylinder is even or slightly above the calipers, put one in anyway. If you don't and you park on a hill, fluid will siphon! These valves are cheap insurance - put them in!
NOTE: You will know if you need one of these valves if you had to pump the pedal twice to get a good pedal. See illustration for more.

10 PSI Valves - These valves are used in a drum brake system to prevent air from being ingested into the hydraulic system when you release the brake pedal. Typical wheel cylinder seals only seal when there is pressure behind them. Rapid release of the brake pedal creates a vacuum in the system which causes the seals to relax and air is ingested into the wheel cylinders. Maintaining 10 PSI in the system at all times prevents this. Some disc/drum master cylinders have 10 PSI residual pressure valves installed internally, some don't. If you're not sure, call us and we can tell you how to check. Also, some new style wheel cylinders have cup expanders which negate the need for the residual pressure valve. Either way, if you are not sure whether you have one or not, put one in. They are not cumulative and it won't hurt anything if you have two. Don't worry about brake drag, it takes roughly 75 PSI to overcome the return springs.


Here is a link to ECI's web site:

http://www.ecihotrodbrakes.com/

Plasticman

larryseed

thanks for all the help.

1955 copper

[QUOTE=Plasticman;1559669623]Thanks

larryseed

Took the valve out of the m/c. Is there a residual valve in the cly on thr hydrovac?

Plasticman

Frankly don't know, but I would doubt it. Boosters should just "boost" the power to the brakes. However, never know until we can look at it. Post a picture or 2.

Plasticman

larryseed

ok I will take a look inside.