Project is stalled. Cannot solve brake problem

[Peter coppola]

OK, My C10 project has been stalled now for several months. I have not figured out my brakes.

Read every bleeding post I could find.

Restoring a 79 C10 Stepside Short Box. Everything is new.

Front Discs (new callipers/hoses)
Rear Drums (new cylinders/lines)
All new AC Delco rubber lines
All new stainless lines from inline tube
New vacuum booster (Cardone)
New Master Cylinder (AC Delco)
New Combo valve. (PV2 Brass aftermarket from Amazon)

I have change new Master Cylinder Three times
I have changed combo valve twice

I have the combo valve locking tool installed in place of the brake light switch from the get go on the latest combo valve.

We have pedal bled the brakes.
We have vacuum bled the brakes with a hand vacuum
We have pressure bled the brakes with a pressure bleeder kit from OTC

With the truck not running, we get a rock hard pedal. As soon as we start up, mooshy pedal to the floor ad repeated pump offers slight pedal.

We have tried bleeding with the truck running, no difference.

We are getting excellent flow out of the two front calliper bleeders.

We get very slow flow out of the two rear cylinder bleeders.

Have doubt there is air in system as we have seen now bubbles under pressure bleed while bleeders are open for minutes with fluid level rising in the bleeder bottle.

We hold over 10 PSI for hours on the pressure bleeder and all lines were snugged numerous times.

This is a stock stepside Short bed setup. Combo valve is under the radiator on a frame crossover in front of cradle. Royal PITA to access.

Would love to get this brake system fixed to move on to other work.

I have 18 psi of engine vacuum in my system when running. New GM Crate 350.

Any other ideas what we can try?

 

[Craig Nedrow]

Is this vacuum canister, or upgraded to Hydra-boost. If vacuum canister, is the diaphragm good? You have plenty of vacuum for a diaphragm system.

 

[legopnuematic]

Are the rear drums adjusted correctly?

 

[jh86]

Did you bench bleed the master cylinder?

I just did the entire brake system on my 86 a couple months ago with almost identical part list. Went smoothly overall.

Bleeding was tricky for a bit. I found the old fashioned method with I open bleed screw, dad press brake pedal, close bleed, pedal up, repeat… worked best. With that had the worst time until I realized I wasn’t opening the bleed screws far enough.

rear pass, rear driver, front pass, front driver. Open those bleed screws up to get a stream. Repeat process several times.

I also used proportioning valve tool

 

[jh86]

Just reread your post. I was also getting low flow out of rear until I really opened up the bleed screws. Solved my bleeding problem

 

[Camar068]

I like the gravity method. Master cylinder cap off, open the bleeder to get some flow. Then tap on components (screwdriver, wrench, etc) to help the bubbles, stuck in higher corners, move out. I've even left one barely cracked open overnight. Ceramic Powerstop pads and new calipers, then gravity bled....never felt the pedal this good in 8 years.

Enjoyed it way more than fighting with the wife using the pedal push method.

 

[Turbo4whl]

If the rear brakes shoes are not adjusted out to the drums, then you will have trouble bleeding the rear cylinders. Remember the (self) automatic adjusters will only maintain the adjustment. When the shoes are way out of adjustment, the adjusters will not take up any slack.

One trick, to bleed out of adjusted rear shoes, apply the emergency brake. This moves the shoes to the drums. Then you can bleed them. If the e-brake pedal goes to the floor and the drums still turn, shoes are way out of adjustment.

 

[DoubleDingo]

I didn't read that you bench bled the master cylinder

 

[mtbadbob]

Sounds like a vacuum issue maybe? Do you have good vacuum to/at the booster?

 

[77Dmax]

Adjust the rears for sure. I'd also drive it before condemning it. I've had plenty with a soft pedal feel that work beautifully.

 

[Peter coppola]

Hello all. Answer to all the questions:

1. New vacuum booster so diaghram is new
2. Rear brakes adjusted properly and parking brake is set and not to floor
3. Each time, I bench bled master cylinder before install

 

[Ricko1966]

Did you adjust the brake booster pushrod properly? Not the rod from the pedal to the master,the one in the booster that pushes on the master?Was this a power brake car to begin with or is this add on. If added on have you changed the brake pushrod pivot location accordingly? No vacuum or a ruptured diaphragm causes a rock hard pedal not a soft pedal.

 

[fast 99]

When bench bleeding a master do not bottom out the piston. It is possible to cut one or more cups doing so. Are the caliper bleed screws the highest point on the calipers?

Ricko1966 is suggesting the master may need a tag wire procedure performed. It's easy to do but takes 2 people.

 

[Ricko1966]

There is a hole in the master cylinder,sometimes there is spacer in that hole also. The brake booster has a pushrod that fits in that hole,and that pushrod is what actually moves the master. You should have .020 clearance between the pushrod and bottom of the hole or the spacer on the master. If the stick out on the booster isn't enough the pedal moves,the pedal pushrod moves,the booster gets moved but the Plunger moves inside the mastercylinder bore,instead of moving the Plunger. Also if the brake pedal pushrod is too short it won't move the booster enough. And also if the brake pushrod is in the wrong location on the pedal it may not move the booster enough. Many vehicles use a different pushrod location depending on whether or not you have power brakes. If you switched from manual to power you may need to relocate the pushrod. Now all three of these may be wrong or even just two of them,and the tolerance stack is keeping you from having a pedal. I can teach how to check the clearance,without special tools,if you haven't already done it. Actually so can @fast 99 .