Hi all. First time posting. I just completed(?) a 4 wheel manual drum conversion to manual disc front and drum rears on my '67. The discs are stock 69-72 A-body. I'm using a new '69 Chevelle manual disc master cylinder, the original '67 distribution block, and an SSBC adjustable proportioning valve on the rear main line after the distribution block (many thanks to those who have posted before me for the tips). The brake lines are all brand new from NPD with the exception of the rear axle rubber hose and axle lines and I replaced all four bleeder screws. The problem I'm having is that the rear drums will not work (both front and rear drums worked great before I started this project). I bench bled the MC (verified that it did bleed both reservoirs) and tried to bleed the lines in the correct order (RH rear, LH rear, RH front, LH front), but the rears wouldn't bleed. In trying the fronts after giving up on the rears, they bled fine (gravity and using the pedal) and work great and my pedal has a nice hard feel (after 'bout 3" of free travel), but the rears won't even stop the wheels being turned by hand. I tried all the bleeding suggestions for the rears in the various other postings. Gravity bleeding wouldn't yield a drop. Mityvac bleeding would get a little bit of fluid at first, then nothing. Normal pedal pressure bleeding would also get a little fluid at first, but the pedal will not soften even a bit much less seep to the floor while pushing with the bleeder screw open. I'm using the upper push rod hole on the brake pedal, so no binding and I've tried adjusting the prop valve from full open to half way to full closed with no change (yes, it's piped in correctly). Just for giggles, I took the rear line loose at the MC and used my Mityvac to see if I could draw fluid out of my new bottle of fluid (DOT 3) through the right rear wheel bleeder screw and up to the MC (to make sure there was no blockages). It worked great - lots of clean fluid. I've even tried a different disc/drum MC from a later model F-body - no change. What am I missing here? Does the '67 distribution block have a piston similar to the later model combo valves that may have shuttled to one side? If so, why would I get fluid using my vac pump pulling fluid from the rear bleeder to the MC? I understand this set up should work from other postings (even if it's not as good as a combo valve kits). Any fix (or problem solving test) suggestions would be greatly appreciated.

P.S. Fantastic web site - especially for Chevelle junkies like me!

BTW.....the Woodward Dream Cruise is less than two weeks away, so I'm starting to panic.

It is possible that the wheel cylinders are plugged up. This happened to me on my 70 Malibu. I put new disc brakes on the front but did nothing to the rear drums. I went to bleed the system and could get no fluid from the rear bleeders. I then re-built the rear brakes - new wheel cylinders, shoes and spring kit. After that, they bled just fine.

I would suspect the wheel cylinders. Replace them, they are inexpensive. Good luck.

Ummm, the "67 distribution block", I am not sure that's gonna be right. The 69-72 car have a block that provides dual ckt control (internal valve to close if either front/rear breakes fail and a ground for the brake light in the dash).

Was the 67 system a single reseviour? If so, that block is just a "t".

Maybe use a combo valve from http://www.mpbrakes.com
I think you need a residual pressure valve to keep the pressure in the rear line.

http://www.911brakes.com/residualvalves.pdf


Ok, move forward. Can you bleed fluid/air on the rear brake line at the master before the valves?

One thing that happens is that you end up pushing the pedal/rod so far forward before the master has much fluid in it that the internal valve(s) gets stuck.

If you get fluid/air at the master for each line, then move back to just after the combo valve. Work your way backward from the master.

I'm thinking the master, so either swap it out or dissassemble it. Oh, you will need to add the residual valve to get the rears to come up to the right presure. :(

I'm no expert but let me see if I understand your post:

- System worked fine before you touched it therefore it can be assumed that the components that remained in the car are probably good.

- "something" is preventing fluid pressure from being applied to the rear portion of the system.

- therefore:
- if the rear activating portion of the master cylinder is exerting pressure, (which is easily discerned by cracking the line while someone presses on the peddle)
-and it's not making it beyond the distribution block, (again easily discerned by cracking the "rear-brake-activating" line fitting, AT THE BLOCK, while pressure is applied to the peddle)
- it would indicate to me that perhaps the block is stopping the fluid. My 67 valve has an electrical connection on it which would indicate to me it can detect an "out of ballance" condition between the front and rear halves of the system. This is usually done with a piston in the valve moving towards the lowest pressure side of the block. I suspect you will find this is a part of your problem. You may want to use a distribution block from a later model car which had a dual chambered master cylinder (has proporting valve built in) or get a dual system valve from SSBC. They not only have them but have a relocating kit too in case you ever want to run headers.

Just my $.02 worth

Eureka! Finally was able to get the rears to gravity bleed last night. In a fit of frustration, I drove up and down my driveway on the front brakes and then tried re-bleeding the rears. I still had some trouble in that I had to play with the bleeder screws (turn them off and on) to keep the fluid flowing. The left side was more troublesome than the right. After checking for fluid at several joints, it's painfully obvious that my trouble is at the rear wheel cylinders. I guess that's what I get when I 'assume' that they would work after the fact since they worked before. Anyway, they're functioning now, but I'm going to put new cylinders on the rear in the very near future. Thanks for all of the suggestions!!!! Hopefully, I'll be able to return the favor one day.

BTW.... '67's came with dual master cylinders and a non-proportioning brass block which I may or may not be correctly calling a 'distribution block'. Herb: My Dad thought the distribution block may have been the problem as well, but we ruled it out when I was able to draw fluid from my bottle through the rear cylinders to the MC line with the Mityvac. I sure would like to understand the distribution block function better. My service manual doesn't have a cross section of the block to see if there's a shuttle valve, but, like you, I believe there has to be some kind of valve to sense the pressure differential in the case of a failure since the light switch attaches to it. The troubling thing is that you would think GM would have some sort of service procedure to get the valve centered again after a failure, but I can't find anything in my manuals or on line that references a service procedure.

Thanks again for all the help guys!

Not sure, but I think the method of balancing the shuttle valve is just to bleed a front or rear wheel alternately until the light goes out. I'm reaching here into some VERY old memories so I'm sure somebody else can correct me :D