Front brakes retaining pressure

[88Ironduke]

My 72' didn't come with power brakes so I added a booster and master cylinder to fit. I've used my stock 72' disc/drum proportioning valve, its pretty grungy looking but seems serviceable. New calipers/pads and fresh bearings have been installed. I changed the brake pedal input shaft to the shorter one for power brakes and everything seemed to be peachy. However I noticed that my truck got hard to push around. I thought the drum brakes were sticking/hanging up but it turns out both left and right front brakes were being applied. I tracked the pressure being held all the way thru the proportioning valve to the master cylinder. If I crack the line at the master cyl and let off a bit of pressure the brakes release as advertised. If the push rod is adjusted out to far will the master cylinder hold brake pressure on the front brakes and not the back?

Thoughts?
Paul

[Wrenchbender Ret]

Yes it can put pressure on the front only. Back the mounting nuts on the master cyl back a little & pull it back. Then try it.

[CastIron]

I've seen this before on newer cars. In that case, it was the stoplight switch adjusted to tight, holding the pedal down just a little. I haven't looked under the dash of my GMC for a while, so I'm not sure how exactly they work on these old trucks.
Maybe pull up on the brake pedal and see if anything changes? That's how I have diagnosed similar complaints.

[cdowns]

do you have the pushrods adjusted properly on the master cylinder and the power booster// check the service manual for proper length adjustment

[88Ironduke]

I have checked the brake input shaft from the pedal assembly today. I started out with giving three turns to shorten the input and checked to see if I still had my front brakes applied and they were. With the input rod disconnected from the pedal and freely hanging I still have front brakes applied. So I put the pedal rod back to where I started from when I installed it. When I assembled the support bracket there were no issues. I bead blasted all bits and pieces, painted and lubed for smooth operations. When I loosen up the master cylinder bolts I immediately get my front brakes to release. The input shaft from the booster to the master cylinder is a fixed length piece of material. Its fully seated in the booster but is definitely applying my front brakes. My initial thought is the rod needs shortened up about 1/8" to 3/16" but I am not a big fan of that option. But since they were purchased off the forum I assumed they were a "kit" since I got them together but now I am not so certain, maybe mismatched from my seller a couple of years ago. Its been a bit.

Penny for your thoughts.

[TR32]

Any resolution? I just put disk brakes on my 69 but they are from a 92 c2500, I haven't changed the master cylinder yet and don't plan on using a booster. I am having a similar issue with the pressure......

[ERASER5]

Originally Posted by TR32

Any resolution? I just put disk brakes on my 69 but they are from a 92 c2500, I haven't changed the master cylinder yet and don't plan on using a booster. I am having a similar issue with the pressure......

You cannot use a front drum MC. There is a 10 psi residual valve used for drums. You may be able to remove the residual valve, but I would bet the bore and stroke of the MC will still be wrong.

[TR32]

If the residual valve resides in the prop valve then that's not the issue. I'm only running front brakes at the moment until I replace the rear wheel cylinders and put the new master cylinder in place. I just haven't had the time since the weather went to crap. Braking is actually pretty good, way better than the drums that were on it. I do understand the difference between master cylinders but I didn't know there was a residual valve in the system. Thanks for the post!

Scott

[gottattooz]

I had an issue like that, but my rear disk brakes were dragging. It turned out that the rod between the master and booster was about 1/8 inch too long. When I took the master off the booster, I put the rod into the booster and marked where the rod sat flush with the seal. Then I stuck the rod into the back of the master cylinder and marked where is sat flush. The difference in the marks was about 1/8 inch. I cut 1/8 inch off the flat end that seats in the booster, assembled everything, and the problem went away.

-Josh

[kbill45@gmail.com]

Try a shorter pushrod

[swamp rat]

I got a different push rod when i bought my new booster and it was a bit longer than the stock rod by probably 3/16, i ended up using the stock rod but i also checked it for clearance with a little ball of modeling clay to be sure it wasn't pushing on the rod when bolted up.

[TR32]

Luckily my push rod is adjustable. Before I left work I shortened it about an 1/8', seemed to do ok but by the time I got to the store there was still too much pressure being applied statically. I took about another 1/8' out of it. No issues after that, maybe put turn back in it. Overall I love the feel of the pedal now. Now when the weather gets a bit better I will swap out the master cylinder so that everything is correct. That being said, a drum/drum MC will work with disk brakes. Is it correct? No, but it stops a hell of a lot better than with drums on the front that where new from wheel cylinders to the drums.........

Thanks for the replies!

Scott