68442Convertible

When I converted my front drums to discs, I decided to change my master cylinder - even though it was fairly new. I picked up a disc/drum MC at Napa. At some point, I read a posting on a hot rod site by an old timer that said that most MCs sold today do not have a check valve in the drum output. He said you could check it by inserting the back of a drill bit in the drum output. If it goes in all the way, there is no check valve. So I checked the new MC I just bought, and there was no check valve in the drum output. Then I checked my existing drum/drum MC - no check valve. Then I drove to Napa and checked several MCs that they had in stock. No check valve.

This has probably been covered before, but I just wanted to make folks that don't know aware.

The check valve is also called a residual pressure valve (RPV) and my understanding is that it maintains approximately 10 psi on the drum wheel cylinder to prevent wheel cylinder cup leakage. Some wheel cylinders use a conical washer at the end of the spring - called a cup expander - and do not need a RPV. I looked at the rebuild kits for my car and they do not show a cup expander, so I think I need the RPV. I can get a 10 psi RPV from Wilwood.

My car is not driven for long periods of time, so I'm not sure how much value the RPV has for me. Seems likely that the residual pressure would bleed down over time. Not sure.

joe_padavano

A residual pressure valve is not the same function as a check valve. The purpose of the residual pressure valve is to take up the slack in the drum brake linkages for shorter pedal travel. If the valve leaks down over time, that simply means that the first time you step on the brakes (to shift into DRIVE), the pedal goes a little farther. Once you start driving, the valve doesn't leak down appreciably.

I cannot comment on whether or not new M/Cs have the valve, but it would not surprise me if the Chinesium replacements available today leave it out to save a few pennies on each one. The factory installed M/Cs definitely had them.

68442Convertible

I have also read that explanation for the use of the RPV.

The wheel cup leakage explanation was used in an article I read to explain why the valves are no longer being installed in MCs.

joe_padavano

If 10 psi is causing your wheel cylinders to leak, you have a bigger problem and leaving the RPV out is just masking that problem.

68442Convertible

Sorry, I meant that the 10 psi is suppose to prevent leakage, not cause it. At least that is what I've read as an explanation.

Oldsmaniac

Just a FYI, recently have replaced entire brake lines on a couple of my project cars, bench bleed the master cylinder and install to find out on the 1st bleed return stroke, the master will suck air from the empty new lines and loose its prime. I always thought there were check valves on the master cylinders but I guess not any more. For me it was required to back feed fluid up to the master cylinder in order not to get air back into the master cylinder.