proportionong valve help please

[meathead95]

i put 72 axles under my 1970 k20 so i took the proportioning valve off of a 72 parts truck for the dic drum part of it. but the problem i ran into is that the front line on the 72 is smaller then on the 70. so what is everyone doing for a proportionong valve? i need to get this resolved asap

[Steven B]

I just ran new front lines. You can either buy them new and prebent from Inline tube or go to the parts store and bend your own.

[meathead95]

i was hoping that wasnt the case lol............

[Steven B]

If I remember they were about $80.

[70c10]

I just went thru this recently and had the same problem. I bought reducer fittings (1 for the front line and 2 for the rear) to attach the existing lines to the new prop valve. I don't know what sizes because I didn't write them down. I made about 5 trips to the local parts stores before getting the last one at NAPA.

[cooters]

since proportioning valves are only there to limit rear brake lockup, i run all my projects without them, in some conditions i run wilwood adjustable ones. The other perk to not running factory ones is if you have a brake line leak you will not lose all of the brakes, you will still have front or rear.

[boraxman]

I figured it would be a good time to upgrade to stainless lines myself.

[meathead95]

i went and bought some at autozone and made my own. but the brakes still arent worth a crap. i wonder if it is the rod adjustment going from the booster to the mastercylinder?

[jacobs]

Are you using your old 70 master cylinder? If so, I think you need to change to 71-72 master cylinder and booster. I battled with a similar problem and finally purchased a 71 booster and master cylinder...works GREAT now.

[meathead95]

yes i have a BRAND NEW 1970 mastercylinder and booster with a 72 proportioning valve. what makes a 72 booster better?

[jacobs]

I don't remember if there's a difference in the master cylinder piston size between the two but I do remember there's different part numbers for both the master cylinder and booster. Maybe I had a defective booster but after I changed both, my braking problems ended.

[Yukon Jack]

I am putting on a 78 3/4 ton's master cylinder on my 70 K5 and am using the 78's proportioning valve. Ran into the same problem of the lines not fitting. Plus my existing hard lines didn't fit the new rubber lines for the 78's disk brakes. I bought some various length straight brake lines and am just going to redo the front brake lines - that way I won't need adapters at the proprotioning valve and at each front rubber line. For the rear line I may have used an adapter, can't recall.

[jacobs]

I now have more time to explain what I went through with my braking ordeal....

My braking problems started when I converted my 70 c/10 to 4x4 using a 73 Blazer as a donor. Originally I just had manual brakes so with the new Blazer disk brakes, braking was reduced. I installed my old 73 Blazer proportioning valve but I couldn’t tell any difference. Then I installed the old Blazer master cylinder but no change. I then purchased a 1970 C/K10 booster from NAPA. It helped with normal stopping but then it was worse for quick stopping and next to impossible to lock up the tires. I tried both the original 1970 master cylinder and the Blazer master cylinder but there wasn’t any difference. Later, after losing the rear wheel bearings and axles the second time as I frequently haul heavy loads, I converted to 3/4 ton front and rear axles. The rear axle has 13" X 2 1/2“ brakes so that helped stopping but it still wasn’t good. Finally I installed a used 1971 3/4 ton booster and master cylinder.....It stops GREAT now. Why? I don’t understand. The 1/2 ton master cylinder has a smaller piston in it so it should give more pressure.

By the way, I’m using my old steel lines with an adapter into the master cylinder.

[c10crazy]

meathead, if you put disc brakes on you need a master cylinder for 71'- up. Your 70' master cylinder is for drum brakes.

[meathead95]

i have always heard that the proportionong valves varied from drum to disc. but i never thought the master cyinder did. I hope i can get this stuff to work as i already have 125$ in the new booster and mastercylinder

[jacobs]

Actually....what looks like a proportioning valve on 4 wheel drum brakes is only an electrical switch that has contacts that close when there is an imbalance in brake line pressure between the front or real braking systems and inform the driver of braking problems.

Proportioning valves are only used with disk brakes.

I've always thought the only difference in master cylinders between disk and drum was just the size of the reservoir....and it may be.

[70c10]

Originally Posted by meathead95

i have always heard that the proportionong valves varied from drum to disc. but i never thought the master cyinder did. I hope i can get this stuff to work as i already have 125$ in the new booster and mastercylinder

The drums use a smaller "junction block" where the discs use the larger "proportioning valve". The MC for the drum brakes is real small compared to the disc brake MC which holds more fluid. Just take your 70' MC back to the store and exchange it for the 71/72 style. I paid around $15 I think.

[cooters]

the master cylinder is way differant for drum to disk brakes. The calipers have a 2 inch piston vs a drum having a small wheel cylinder. you will never get enough volume of fluid to the front to properly move the caliper piston. I know many people who left the prop valve from the drum brakes in when converting to disk with no problems. As far as fittings go, i go to a local pick n pull and find fittings from other cars that match what i need.

[meathead95]

Originally Posted by 70c10

The drums use a smaller "junction block" where the discs use the larger "proportioning valve". The MC for the drum brakes is real small compared to the disc brake MC which holds more fluid. Just take your 70' MC back to the store and exchange it for the 71/72 style. I paid around $15 I think.

so i take it a 71/72 style mc will work on my 70 booster just fine then? I just figured they held more fluid. I never thought they would supply more pressure. but if yall think thats the problem then ill go get a different one.

[VetteVet]

Here is an excellent tech article that should explain all about disc brakes.

http://www.hotrodheaven.com/tech/bra...kes1_index.htm

[meathead95]

Thanks, I guess Ill try a 72 mc and see what happens.