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Author Topic: Disc Drum Brake Master Cyl  (Read 3835 times)
red69
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« on: March 30, 2009, 09:28:55 AM »

My son changed his 67 over from drum drum to disc drum. While doing so he changed the master cyl. He is now having trouble with no or very little brake peddle. I reseached the site and suspect he has no check valve in the drum portion of the master cyl. Does anyone make the proper cyl? Or do you have to install the check valves yourself? The new one he bought was supposed to be for the application, I'm not sure of the brand.
   Thanks Pat
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red69
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« Reply #1 on: March 30, 2009, 01:56:42 PM »

Update, I checked the old master cyl. which had been working fine with the drum brakes and it has no check vavles in the outlets. How can that be? I thought all drum systems had them.
   Pat
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JohnZ
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« Reply #2 on: March 30, 2009, 03:10:37 PM »

Update, I checked the old master cyl. which had been working fine with the drum brakes and it has no check vavles in the outlets. How can that be? I thought all drum systems had them.
   Pat

All original drum/drum master cylinders had RPV's (Residual Pressure Valves) in the outlets - they're located behind the brass seats (you have to remove the brass seats to see them). If that master cylinder doesn't have them, it's probably an (incorrect) replacement. See photo below - (1) is the RPV spring, (2) is the rubber RPV, and (3) is the brass tube seat.
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'69 Z/28
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red69
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« Reply #3 on: March 30, 2009, 04:02:05 PM »

I understand what you are saying John. This has me real puzzled. The car is a 02 production 67, the master cyl. has Delco M cast into the case, and it has a date of H-1-6 cast into it. The master cyl is not going back on, so I put it in the drill press and drilled out the brass seat, there was nothing behind it. Assuming that someone rebuilt it and left the check valves out, how would the car have had good brake pedal before it was dissasembled? My son has owned the car for several years and it never had brake issues. I'm stumped
   Pat
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rat pack
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« Reply #4 on: March 31, 2009, 01:56:27 PM »

Do both master cylinders have the same depth for the pushrod coming from the pedal? I have seen rebuilt master cylinders get the wrong piston and the pushrod needs to be changed. The car wouldn't really start to stop until the pedal got closer to the floor. Also I would assume he bled the master cylinder before installing it, and did he bleed the brakes after he was finished? ............RatPack.............
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Just keep livin......L I V I N .............
red69
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« Reply #5 on: March 31, 2009, 05:11:17 PM »

We've checked pushrod depth and bled twice, will go through all again tommorrow
  Thanks Pat
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rich69rs
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« Reply #6 on: March 31, 2009, 06:10:08 PM »

Pat,

...some random thoughts in no particular order-

1)  If you search the site, there have been several posts in the past dealing with rebuilt / new / replacement master cylinders and the lack of rpv's being installed as required for drum brakes (disc/drum or drum/drum) by the rebuiler/remanufacturer leading to similar issues as you describe.

2)  Power or manual brakes?  Manual brake master cylinders set more or less leverl and generally do not have bleeder valves.  Master cylinders for power brakes generally will have bleeder valves in order to facilitate bench bleeding which is required due to the master cylinder being mounted on an incline.  The attached picture, although not the best quality, shows the original master cylinder in my '69RS for power drum/drum.  Note the bleeder valves on the master cylinder.  If your setup is power disc/drum, your new master cylinder should have bleeders. 

3)  As JohnZ stated, since you still have rear drums there still should be a residual pressure valve in the master cylinder for the rears. 

4)  Are you still using the same type of brake fluid?  Reason I ask is that I changed the fluid in both my Chevelle and Camaro to DOT5 (silicone).  There are some advantages, but one disadvantage is that DOT5 can be harder to bleed.  Also, be sure that the rear drum shoes are adjusted out properly.

Good luck - let us know what you find.

Richard
« Last Edit: March 31, 2009, 06:40:07 PM by rich69rs » Logged

Richard Thomas
1969 RS
red69
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« Reply #7 on: March 31, 2009, 08:25:05 PM »

Thanks
All good things to check, we'll figure it out. Gotta love brake bleading problems. I'll post the results when we get it
   Pat
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red69
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« Reply #8 on: April 01, 2009, 08:30:42 PM »

Still locked in brake HELL. We pulled the master and re-bench bled. Re-bled all wheels twice, no better, blocked off line to rear wheels, no better. I guess we are going to try another master cyl.
   Pat
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tom
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« Reply #9 on: April 01, 2009, 08:58:54 PM »

Power or manual?
Did you switch to silicone? I have read (here) if you switch to silicone with any old fluid still in the system you will have problems.

Are the calipers on the correct side? The bleeder should be at the top of the caliper. If you put the caliper on the wrong side of the car, you can not bleed it correctly, and air remains inside the caliper.
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69 X11 Z21 L14 glide
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red69
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« Reply #10 on: April 03, 2009, 09:39:13 AM »

The brakes are working! Very strange finding, maybe someone can shed some light on this. I decided to start comparing brake linkage with my 69, my son's car is a 67, both cars have power brakes the 67 was drum-drum, we switched to disc fronts and changed master cyl. My 69 is a disc brake car. The push rod on my son's car was in the upper hole on the brake pedal, the 69 was in the lower hole, which the assy. manual says is for J-50 option. The 67 had a return spring on the linkage which is for manual only I believe. I figured someone along the way must have converted it to power brakes, and not put the push rod in the proper hole. The strange part is he has been driving the car for years, I have driven it also, and it had good brake pedal with the old system.
Anyway it has pedal now and stops fine.
 Thanks for all the info     Pat
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JohnZ
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« Reply #11 on: April 04, 2009, 09:55:18 AM »

I figured someone along the way must have converted it to power brakes, and not put the push rod in the proper hole.

Common mistake - with manual brakes, the pedal pushrod goes in the upper hole, and with power brakes the pushrod goes in the lower hole, so the push geometry matches the mounting angle of the master cylinder (or booster).
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'69 Z/28
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