The Stovebolt Site Forums The Shop Area The HiPo Shop Brake dilemma

Ok putting my 52 3100 back together. In the process of adding under floor power booster and dual master cylinder. Have bled new master cylinder and have fluid to all four corners but brakes will not build pressure. What could be the problem? No motor in the truck so doing everything without vacuum, shouldn't I still get pedal pressure? Also all new hard lines and rubber lines.

Problem with an under the floor master is that it can end up being the low spot in the system.
Mounted up on the fire wall, there is positive pressure in the lines due to head pressure in the lines. If the master is mounted it the level of the wheel cylinders or have lines running higher than the cylinder, fluid runs back and there is no residual pressure in the wheel cylinders.
Install a residual pressure valve in each line, close to the master.
Secondly, With lines higher than the master, air traps are common. Pump and bleed won't flow enough to clear them out. You have to use a pressure bleeder. Rent or make your own, not difficult.

4 wheel drums? How did you bleed the master- with plugs or with the tubes to recycle the fluid, on a bench or in the truck? I would look first at the master- if it's not defective or damaged, then it might need to be re-bled at a different angle to get all the air out. As Jim said, you can get air in the system. Occasionally, raising one end or the either of the truck can help to solve that problem...but not always

Do you have a proportioning valve setup? Did you install the dummy plug in the valve to bleed it properly? Need more info on your system to help...

discs or drums
maser cylinder bore size
proportioning valve or not
residual pressure valve(s) ( RPV ) installed or not

First I would screw block off plugs in the master cylinder where the lines go. The pedal should be "locked" if mc is good. Next try reverse bleeding since your wheel cylinders are higher than the mc; put a catch pan under the mc. 20psi is enough to over power any residual pressure valves. Reverse bleeding can be messy so we use an old Wagner bleeder ball where a diaphragm pushes down on the fluid in the ball and forces a pressurized all fluid,no air, stream through the lines. With a two gallon capacity one can leave a bleeder valve open till the stream is completely clear. I don't know if one can rent these but if you can I'd nearly bet the problem will disappear.

The system is disc from drum rear, has proportioning valve installed, no residual valves though. Not sure of the master cylinder bore size.

NorCal Dave, I bled master on the truck with it levelusing the tube back to reservoir.

Thanks Coilover, will try plugging ports where lines go to master and then to proportioning valve.

Just covering all the bases- when you say it was "level" while bleeding, did you use a level on top of the reservoir, or was the truck just sitting on level ground? You probably already know this, but the upgraded MC setup usually isn't level when the truck is level. It sits at an angle that traps air in the MC bore.

NorCal Dave yes, I put a level on top of reservoir to check level, had to raise the front of the truck quite a bit to do that.

I'd do the test Coilover suggested with the plugs. If it passes, then I'd look at pressure bleeding, as Jim suggested... I have a setup similar to yours and I still have some air in the system and will have to pressure bleed as well. I did install RPV's front and rear though. Good luck.

It is pretty easy, if one has access to a lathe, to make a pressure bleeder cap with some appropriately sized 'O' rings to pressurize the master, then bleed slaves one by one.

Ed

Have you tried adjusting the master cylinder push rod? If not, just change the length in small increments, too much and the fronts will drag.
That was the problem for me when I installed a dual master cylinder.
don

Don, I don't know if he can do it that way if he has a booster..?

Dave:
I have a CPP dual master cylinder with booster. The push rod is adjustable. I only know about mine, I have no experience with any others.
don