Valve in new master cylinder - The 1947 - Present Chevrolet & GMC Truck Message Board Network

**MikeB** · 11-06-2010, 09:36 AM

When I converted my 69 C-10 to front disc brakes 15 years ago, I used a complete GM power brake setup from a 71-72 C-10 -- brackets, pushrod linkage, booster, M/C, and proportioning valve -- that I bought at a junkyard. As I recall, the master cylinder was bad, so I bought one for a 71-72 at a parts store. The brakes have worked well over the years, but the master cylinder and brake lines are crusty rusty, so I've decided to replace them.

I just got a Wagner master cylinder P/N MC80568 which is supposedly a direct OE replacement. However, I noticed it has a spring-loaded device in the rear port. (When I push on it with a drill bit, it springs back.) I assume this is a residual valve, but why would it be there when all the 71-72 trucks came with a proportioning valve? Is the internal valve required? If not, can it be removed?

I haven't removed the old master yet, so can't say whether or not it has the valve.

Thanks for your help here.

raycow · 11-07-2010, 10:51 AM

Any drum brake needs a residual pressure valve to prevent a low pedal condition caused by the shoes retracting fully. A proportioning valve is not a replacement for a RPV. It is possible to combine the two functions in one housing, but then it is called a combination valve. Long story short, unless you can positively verify that you have a combination valve, leave the RPV in the MC.

Ray

11-06-2010, 09:36 AM	#1
MikeB Senior Member Join Date: Jul 2000 Location: North Texas Posts: 3,444	Valve in new master cylinder When I converted my 69 C-10 to front disc brakes 15 years ago, I used a complete GM power brake setup from a 71-72 C-10 -- brackets, pushrod linkage, booster, M/C, and proportioning valve -- that I bought at a junkyard. As I recall, the master cylinder was bad, so I bought one for a 71-72 at a parts store. The brakes have worked well over the years, but the master cylinder and brake lines are crusty rusty, so I've decided to replace them. I just got a Wagner master cylinder P/N MC80568 which is supposedly a direct OE replacement. However, I noticed it has a spring-loaded device in the rear port. (When I push on it with a drill bit, it springs back.) I assume this is a residual valve, but why would it be there when all the 71-72 trucks came with a proportioning valve? Is the internal valve required? If not, can it be removed? I haven't removed the old master yet, so can't say whether or not it has the valve. Thanks for your help here. __________________ Mike 1969 C10 LWB -- owned for 34 years. 350/TH350, 3.08 posi, 1st Gen Vintage Air, AAW wiring harness, 5-lug conversion, 1985 spindles and brakes. 1982 C10 SWB -- sold 1981 C10 Silverado LWB -- sold, but wish I still had it! 1969 C10 (not the current one) that I bought in the early 1980s. Paid $1200; sold for $1500 a few years later. Just a hint at the appreciation that was coming. Retired as a factory automation products salesman. Worked part-time over the years for an engine builder and a classic car repair shop. Member here for 23 years! This is the very first car/truck Internet forum I joined. I still used a dial-up modem back then! Last edited by MikeB; 11-06-2010 at 09:54 AM.

11-07-2010, 10:51 AM	#2
raycow Registered User Join Date: Mar 2003 Location: San Francisco, CA USA Posts: 2,454	Re: Valve in new master cylinder Any drum brake needs a residual pressure valve to prevent a low pedal condition caused by the shoes retracting fully. A proportioning valve is not a replacement for a RPV. It is possible to combine the two functions in one housing, but then it is called a combination valve. Long story short, unless you can positively verify that you have a combination valve, leave the RPV in the MC. Ray __________________ Live more responsibly. Eat lower on the food chain. Eat vegetarians.



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