I recently completed the front drum to disc swap on my 70 GS. new spindles, calipers, rotors, pads, master cylinder and bled the system multiple times. The pedal is still spongey. The car has a factory proportioning valve, but is it a different proportioning valve on a disc/drum car vs a drum/drum car?
Yes it is different. I believe the terminology is one is a distribution block, the other is a proportioning valve. You need to get the correct disk/drum valve. The factory setups can be found, but are tougher. I went with an aftermarket one from inline tube and it has been flawless. If you're a stickler for factory correct, the aftermarket one doesn't mount down on the frame like the factory did as they were configured a bit different, so something to consider. It mounts under the master cylinder. Here's a few pics.
Lack of a residual pressure valve for the rear drums? I had that issue with my disc conversion last year. Another thing worth checking is that the calipers & pad are square to the disc. Had that problem too. I used levers to 'massage' the caliper brackets & get the calipers square to the disc.All good now.
Thanks guys. The pads were really tight on the rotors at first, could barely turn them. A drive around the block seemed to seat them and they are free'd up now. I can only assume that the proportioning valve may have something to do with that as well. I have ordered the aftermarket valve. Looks like I will have to modify the brake lines a little to accommodate. I really had hoped to keep the factory look. Maybe I can mount it down near the original location somehow.
I was able to mount my prop valve on the frame,in a fresh hole. I attached the hard lines to the prop valve while it was not mounted. Then c-clamped the valve to the frame. Drilled a fresh hole. (ouch)
Does the valve even need to be hard mounted? I have seen several examples where the valve is suspended below the master cylinder secured only by the brake lines.
I personally would hard mount it, but then I'm funny about things like that (the "when in doubt, make it stout" mentality I suppose). Does it absolutely have to to function, I guess not.
1970 was the year they had the hold-off valve mounted at the master cylinder. Inline Tube sells them. Sounds like that is the only thing you're missing. The distribution block on the framerail is exactly the same for disc/drum and drum/drum cars that year.
Hi, The system you installed should work, but you would have to remove the original valve on the frame. Then run a straight line from the new valve under the master cylinder to the rear differential. The system will not work with two valves in the system. You can use the original valve on the frame but you would have to remove the valve under the master cylinder and install a 1970 hold off valve as stated in prior threads. The original frame valve is the same for disc and drum brakes in 1970 but you need both for the system to work. The difference is the master cylinder and hold off valve between disc and drum brakes. Good Luck! Ron
Yes, you should mount the proportioning valve solidly to the frame or the master as shown. If you don't, it will flex the brake lines and eventually you will get a fatigue crack and no brakes.
I have never seen a hold off valve before or even heard of such a thing. Anyone have a picture of one? Where can I get one?
Not to steal the Thread here but I did the same thing with my '65, Front Disc brakes. How do you know which Master Cylinder to use, 1" or 1 1/8" ? My pedal feels spongy and it almost goes to the floor. Thanks. Jim
I ordered the valve from inline tube. Will update once installed. Right now the pedal is very spongey and goes right to the floor even after multiple bleeds.
Brent, Here are some picture of the hold off valve. This mounts to the left hand bolt on the master cylinder to booster. Also, see how the line are run in the pictures. The line that come off the side of the hold off valve and rear on master run down to the valve on the frame. The hold off valve is used with the original valve that's mounted to the frame rail in 1970 disc brake set up. I hope this helps! Thanks Ron
Hi Jim, I am not well informed about 65 GS's. I would deferred to some of the 65 Guys. Sorry I couldn't be more help! Good Luck! Ron
Thanks Ron. Much appreciated. So other than for the sake of originality, I could have ordered a proportioning valve from a 71 and eliminated the hold back valve altogether? Also, Is the Master cylinder different for drum/drum vs disc/drum? I ordered the Disc/Drum OEM and it looks different than yours. My old Drum/drum version looks closer to yours and if I could I would prefer to swap it back. The difference being that my old has equal volume reservoirs to both F/R where as the new has a much smaller reservoir for the front than the rear.
Brent, Yes, for question one. Yes for question two, the master is different. The one you have in the picture is the same for 71 and up. The drum master looks similar but the cylinder is smaller in size, bore and volume. The originals masters cylinders from 1970 have the same size reservoirs front and rear, Buick changed them in 1971. I hope you get you issue fixed and get that Buick on the road where they all belong!:TU: Ron
