68/72 brake issues

[Raymond Thompson]

My chassis and drum brakes are 1968(new wheel cylinders and shoes) - the cab and power brake unit are 1972. I kept the 68 proportioning valve. I'm not getting any fluid pressure to the rear wheels. Have I created a monster, or do I just have a clog in the line? Any advice... other than "switch to disc brakes".
Help,
Raymond

[Dark Knight]

I've got the same setup on my '67, but I have disc up front. All 4 brakes are from a '74 Chevy truck, but the distribution block is for 4-wheel drums. It's possible that you have a clogged master (assuming it's used), a clog in the line, or a clogged distribution block. If the MC is used, try a rebuild kit. While you've got it apart, blow some compressed air through the rear lines with the bleeder screws open. Oh, and make sure you clean up any brake fluid!

[Longhorn Man]

why rebuld the master cylinder when they are less than 20 bucks to buy?
Raymond, I would try gravity bleeding them, just open the 2 rear bleeders, and then walk away for 5 min. Come back and see if you have some dripping going on.
If you do, then tighten the bleeders, check the master cylinder level, then (with the cap on) stomp on the pedal like you're doin 100 MPH and a truck just cut you off. That'll reset the valve in your distribution block.
Being that you ahve a distribution black instead of a proportioning valve, your front brakes may not work very well. Disks take much higher pressure to work and if the pressure is even front to back, then you'll have hardly any front brakes, and locked up rears.

[chris71]

if you have 4 wheel drum you need to have a drum/drum master cylinder, that 72 disc/drum MC will not work. Andy, i think you have it a little backwards, discs require more volume and less pressure than drums, drums need more pressure to overcome the springs in their design. Discs need 2 lb residule valves and drums need 10 lb residual valves in the MC. So, Raymond, you are sending too much volume and not enough pressure to the front drums, this is dangerous. I agree with Andy and DK, try and blow some air through the lines, and bleed them, i have heard of the rubber hoses can swell up over time and restrict flow. this may be another item to replace. your 68 distribution block should be fine if it is working properly. good luck.

[Raymond Thompson]

Thanks Chris. I put a drum-drum master on it yesterday. I now have fluid to the front wheels but nothing to the rear. I can not get any significant pedal pressure. I would have thought that if there was a blockage in the rear lines that I would be getting pressure and the front would be working fine... I'm going to blow out the rear lines today, and check that rubber hose that goes over the differential.

[Raymond Thompson]

Thanks Andy. I tried the gravity drain with no luck, and put a new master on... (drum - drum) I haven't got enough pedal pressure to try the brake slam trick. Are you calling the proportioning valve a distribution block, or referring to something else. I reused the proportioning valve from the 68.

[theyammieguy]

It sounds like the new mc might work but since you never had fluid going to the rear then its gonna take forever to bleed them. Try using the vacuum pump method

[craig24671]

I had the same problem with mine. changed to disk on front using 68 distribution block. Changed out the rubber lines at brakes and my problem was solved. I'd be willing to bet that is all you need to do. Hope this helps.

[Longhorn Man]

Are you calling the proportioning valve a distribution block, or referring to something else. I reused the proportioning valve from the 68.

I'm saying there is no such thing as a proportioning valve on a 68, it is a distribution valve.

[raggedjim]

I had that on my blazer once, if I remember correctly it was one of the rubber lines.

I have also had a rubber line do the opposite (keep pressure on a disc brake after the pedal was released) It would lockup the front wheel. The line would pass fluid easily one way but not the other, kinda like a heart valve...

Good luck and check those hoses, Rg

[Longhorn Man]

as far as me "having it backwards"... I know for a fact that I don't. Disks brakes need considerably higher pressure to move the piston(s) verses the pistons in a drum brake. I am unable to find any 'online proof' right now, and I stopped looking after 2 min becouse it ain't worth my time.
When you get an adjustable proportioning valve, you put it on the rear line and restrict flow.
When you bleed brakes, the rear shoots fluid with a little force, the front shoots fluid with major force.
a larger piston is naturally going to be harder to push due to friction alone... calipers need more pressure.

plus, this was the literally day one info in school on brakes.

[chris71]

Originally Posted by Longhorn Man

as far as me "having it backwards"... I know for a fact that I don't. Disks brakes need considerably higher pressure to move the piston(s) verses the pistons in a drum brake. I am unable to find any 'online proof' right now, and I stopped looking after 2 min becouse it ain't worth my time.
When you get an adjustable proportioning valve, you put it on the rear line and restrict flow.
When you bleed brakes, the rear shoots fluid with a little force, the front shoots fluid with major force.
a larger piston is naturally going to be harder to push due to friction alone... calipers need more pressure.

plus, this was the literally day one info in school on brakes.

Okay, so now i am confused:

How to build Hotrod Chassis By Timothy Remus page 97, paragraph 5
“The first little fly in the ointment when building a combination system (disc/drum) is that drum brakes require more pressure for their initial application than do disc brakes. Drum brakes, with their big shoes, return springs, and relatively large distance between the shoes and the drum, require approximately 125psi to actually push the shoes against the drums with enough force to slow down the car. Disc brakes, however, need only about one-tenth as much pressure to push the pads against the rotor with enough force to affect the car’s speed.”

http://books.google.com/books?id=T7j...esult#PPA97,M1

I wasnt quite clear on the residual valves, 10 lbs isnt how much pressure drums brakes need, its the amount of pressure that is kept inside of the line after a depress of the brake peadal, to keep pressure on the cups of the wheel cylinder, so that it wont leak, calipers require less.

Even if discs require more pressure, Raymond still needs a drum/drum MC because the excessive pressure from a front disc MC will wear the pads out real quick.

[chris71]

Disc brakes need more "volume" than drum brakes. Think of the size of the piston in a disc caliper (very large), then the respective size of the 2 pistons in a wheel cylinder (both small). It takes alot more volume to move one huge piston than two small ones. Plus when your pads wear down alot the piston of a disc caliper has to move out more in its bore to take up the slack of the thinner pads, so you dont have a big time lag when you hit the brakes with thin pads. This forward position of the piston in the bore with thin pads means less fluid is in the MC, that is why the front resevoir is bigger on disc/drum MC's.

[chris71]

Raymond, keep us posted on the situation, i hope things are working out good.

[Raymond Thompson]

OK - Here's the outcome to this little saga... The proportioning valve, aka distribution block, has a valve inside that is intended to shut off a front or rear line if a rupture occurs. If you take the sending switch (for the brake idot light) out and slide the valve back to the middle, then fluid passes to both front and rear. After doing this I had very weak brakes to both front and rear. Then I replaced the brake booster and now I'm in business. Fairly good brakes to both wheels.

Thanks to everyone who chimed-in, and I hope this little tid-bit is helpful.

Raymond