69 4 drum power booster tag and 2gv carb spring

[Kelley W King]

I have power drums on my 66 GTO and my 64 corvette. Not as good as disc but they work well for my driving. Had them on my 69 Camaro convertible and they did not seem to work as well. Maybe just not adjusted right.

[rich69rs]

Richard, so what does a factory residual pressure valve look like?  I pulled the brass seat out of an old drum master cyl (rear port) and there was nothing there!  Now I am confused.

The residual pressure valve & spring should be behind the seat.  Refer to the green highlighted text in the 1st & 2nd attachments and middle picture, left column, 2nd attachment.  When you pulled the seat out, it should have been there.

"Another variable at the master cylinder outlets is the use of check valves. They are installed under the tube seats in outlets which are to be connected to drum brakes. The retractor springs in drum brakes tend to force the fluid back into the master cylinder. The function of the check valves is to maintain a line pressure of 8 to 16 psi to keep drum brakes ready for quick responses."

Also refer to:

http://www.camaros.org/brakevalve.shtml#rpv



A general FYI - if you ever find yourself in the need of a replacement MC and you try to source one from any parts source - the residual pressure valves may not be there and the folks at the parts house probably won't know if you ask.  Consequently, my suggestion would be to rebuild a correct MC as opposed to trying to replace it.  Rebuilding it is not difficult at all to do.

Richard

[GMAD_Van Nuys]

Different master cylinders were used for Camaros with manual and power drum brakes per CRG as there were bleed valves used on the master cylinder for power drum brakes:

http://www.camaros.org/suspen.shtml#Brakecomp

[KevinW]

I pulled both seats out of an MD drum master, no valves.  Next I will try a 310 BS master.

[rich69rs]

Different master cylinders were used for Camaros with manual and power drum brakes per CRG as there were bleed valves used on the master cylinder for power drum brakes:

http://www.camaros.org/suspen.shtml#Brakecomp

Yes, power assist master cylinders are mounted at an angle with trapped air pockets necessitating the need for bleed valves - but all drum brake master cylinders be they manual or power assist have residual pressure valves.

An interesting sidebar point from CRG report......power assist drum brake master cylinder is "usually" stamped BS, however...... 

"GM documentation confirms that drum brake master cylinders are BS code and most drum brake master cylinders are coded BS. But a limited number have been observed with the CT code, in both manual and power drum applications. The CT code was used on drum brake applications on Pontiac and Buick vehicles.  It is presumed that the CT part was used as a substitute during a parts shortage that and the differences between the BS and CT master cylinders are minor."

The power assist drum brake master cylinder in my '69 RS (Norwood 01C) is a "CT" stamped master cylinder. 

Richard