68-69 Proportion Valve Front Disc, Rear Drum?
 

The 68-69 proportion valve that mounts to the master cylinder on disc brake cars, is this needed? My car also has the brass valve mounted to frame where the brake light switch is plugged in, is this a proportion valve too? The valve that mounts to the master cylinder leaks, I'd rather eliminate it and just have the one on the frame. I don't think 70 and up used this valve? Would I have to get a different proportion valve if I remove it? Will the one on the frame work without it? My 69 GTO has power front disc and rear drum brakes.

Thanks,
Eric

Eric,

The 68/69 "valve" on the frame is just a distribution block with the low pressure sensor for the brake light and has nothing to do with pressure regulation between front and rear. You do need that valve up on the master. The reason the later models don't have it up on the master is that the proportioning valve was moved down to where your distribution block is incorporating both functions into one piece. You can get one of the later versions but you will have to change your brake lines as the prop/distribution combo piece is completely different and the line positions/orientations are different.

The bright side is the later versions are quite a bit easier to find and much cheaper than the master mounted ones if you're not worried about originality. I think I may have a few kicking around in a box out back.

Later,
Dennis

Thanks Dennis,

Does the 70 and up proportion valve mount in the same location as the 68-9 distribution block? My lines are all new. Are the fittings the same? Are the lines in the same area to were I might be able to bend and tweak to get them to work? I may be interested in a later version if you want to part with one.

Thanks,
Eric

Eric, actually I'm near positive 70 was the same as 68/69. The 71 and up valve mounts on the frame in the same area however the bolt pattern is different as that combo valve is much bigger. It also bolts to a bracket that bolts to the frame instead of directly to the frame like the 68-70/71-72 non power block does. You can get away with just using the front bolt hole on the newer powervalve to go to the frame hole where the old one mounted. In some cases that's more desirable anyways since mounting it on the bracket brings it closer to the exhaust manifold and could interfere if you've got headers on it. The fitting sizes change for the front line from master to valve and the angles change for the front left and right valve to wheel lines. If you bought new prebent lines you'll have to modify everything to use the newer style valve. here's a side by side comparison of them.

The unit on the master is not a Proportioning valve it's a hold off valve that allows the rear brakes to apply before the fronts to prevent nose diving during hard braking.

The unit on the frame is a distribution block on these years that divides the system up and turns the light on when pressure is lost on one end.

The 71 and newer units combine the hold off valves with the distribution block making them a true prop valve.

I've used them on every conversion I've done from drums to disc but have moved them away from the stock location to get away from exhaust heat.

Mike

I bent up and flared the lines myself. Looks like I should be able to bend them enough to get them to line up. Are the proportion valves the same for all GM A-bodies? A friend has a couple 71-2 Chevelles with power discs and the local salvage yard has three 71-2 Cutlass. As fars as years I was just going of how PY has them listed in the catalog, they list just 68-9, don't list 70. Thanks Dennis and Mike for your help.

Eric

yep, the chevelle/olds/buick's are the same.

The 71-72 bracket to mount the combination valve on the frame is available from Inline Tube for about 25.00. If you are doing a conversion on a 68-70 and want to keep the valve in the same location as the factory distribution block, you simply buy lines for a 71 disk/drum car and the bracket. The hold off valves are about 69.00 from m ost vendors...

To keep things simple you could just replace the hold off valve (prop valve) at the master cylinder with a stock 68 new reproduction one. Inline has the repro valve.

The valves are not all the same - they are very year specific and the fitting sizes change from year to year. These brake system work great with the righ combo of parts and not as good with a mess of different year parts.

67-68 had 4 piston calipers so the valving was different. 69 had single piston but still the 2 part rotor, 70 actuall had 3 valves - don't forget the correction valve in the back of the car. 71-72 joined the pieces together only on disc cars. There are also different spring rates in each of the valves.

here is a good link to tell you the history and the difference in the valves.

http://www.inlinetube.com/install%20...y%20Valves.htm
http://www.inlinetube.com/images/art...e%20new/a1.jpg

Thanks everyone. Ok, my 69 has the tall spindles, calipers, rotors and brake lines from a 78 Firebird. Will this work ok with the stock hold off valve? I may just buy a new one, then I don't have to mess with lines.

Thanks,
Eric

Work and function correctly are 2 different things - the car will stop with no valve - how good ? - now you have opened a new can a worms.

the spindle -
1. now you can no longer use the stock upper a-arm and have to go to an afthermarket one $500+/-
2. what brake hose do you plan to use the 78 caliper may be metric.
3. hope you are not running stock ralley wheels - they wont fit - and since the rotor is metric - either will the lug nuts.
In the late 70's they tried this thing called the Metric System - but just for a couple of years and then changed it back - It was just long enough to introduce all these metric parts for added confusion.

Fall all the money youe will have to make this system work right - you could buy the most expensive stock system on the market "NEW" and be cheaper.

I guess you have to decide if you are looking for cheap disc brakes or turning this car into something it is not - 78 firebird.

^^

one thing to note, the rotors do not have metric threads.

all 2nd gen front rotors have standard thread pitch. only two part numbers for them with the difference being a larger outer wheel bearing which coincided with the 78 model year, IIRC.

what IS metric on the brakes is the caliper banjo bolt. but since the calipers have the same mounting from 70 to 81, youre free to use whatever you desire.

the orientation of the brake hose varied as well, but i believe that also came to pass when they switched from standard to metric banjo bolt.

my memory isnt what it used to be, but i think doing the F-body tall spindle swap required the use of grand prix brake hoses and/or outer tie rod ends.

Tubular upper a-arms were $200 from a local street rod chassis builder, using stock lower a arms. All parts are new but for a 78 Firebird. All parts bolted on fine. All metal brake lines were bent and flared by me. Everything is together and on the car. I'm using 15" x 7 ralley wheel. The distrubution block is the factory one. I just need the hold off valve, as mine is leaking. If the stock style hold of valve will work with what I have, I'll buy a new one, if not I'll switch to the 71 and up proportion valve. The only thing original to the car is the transmission, so I'm not worried about keeping it original. So will the stock style hold off valve work?