Hello,

I have a 73 f100 4x4 just put new drums and shoes all around, also new mc,
I bled the system and adjusted the rear brakes to best of my knowledge and I have real spongy brake pedal and the fluid in mc will suck down on rear brake port and be pressurized on front brake side. Any help or suggestions would be greatly appreciated.

 

what are you saying here??? I don't understand your description...

 

It sounds like your master cylinder is worn out.

 

okay, sorry that is kind of confusing.

the master cylinder has two sides to fill one for front brakes and one for rear brakes the one for the rear brakes is draining down and seems to be pressuring up on front brake side when you pump the brakes, and it is a new master cylinder. I was told by another person that the rear brakes may still be out of adjustment so i will try that. I was also told I might need to put a manual adjusting proportioning valve on it.

 

did you bench bleed the master cylinder and get all the bubbles out of it. i believe the way that brake system works is the rears work first and set up how much pressure the fronts get. try turning the little starwheel in the bottom of the back shoes. make it so the shoes are closer to the drum

 

alright... so what I think your saying is that the front circuit is building pressure but the rear circuit is draining the reservoir. Is that correct?

Where is the fluid going? Trace the brake lines... you must have a loose fitting somewhere that is leaking the fluid out... or it could be leaking at the wheels. But either way, you should be able to find a spot that is wet with fluid if it is "draining down" out of the reservoir.

Adjusting the rear shoes can fix the pedal feel, but it will not fix a fluid leak. Find the leak first, then worry about adjusting the shoes.

And the Prop Valve is probably not the problem either (unless its leaking)... the prop valve reduces the pressure to the rear brakes. It might be bad, but that wouldn't cause the reservoir to empty (again, unless its leaking fluid).

And to off-road-dude... both circuits will build pressure at the same time. The primary circuit (the outlet closest to the booster) feeds the front wheels (on a front-rear split, which trucks are) so in theory that one builds first. But in actuality, you build them both at the same time. If one circuit is leaking, you will collapse that spring first and then build pressure.

 

Yes the front circuit is building pressure the rear draining down but no leaks it is pressuring back to front circuit, when you take off master cylinder cover it is drained down on rear circuit and pressured up so much on front that it will gush out when taking off cap.

Sorry for hassle, thank you for the comments.

 

I saw that you have replaced the master cylinder already but the only way that fluid can be moved from the back circut to the front circut is if it is slipping past the valving in the barrel. You probably have a rebuilt master cylinder if you bought it aftermarket. I am guessing whoever rebuilt it scabbed up one of the valves when they put it back together. I would take it back to where you bought it and get another one.

 

ahhhh... I see... like Grizz says, you have a bad master cylinder. The separating seal that is located between the primary and secondary circuits is leaking. Hopefully you recently put the new MC on and can take it back and get it exchanged for free.

 

ya it is a refurbished mc from checker auto parts, so I will take it off and try to exchange it. Thank you very much.

Hopefully this works, I bought this truck about 2 years ago completely stripped down to frame and have been putting it together and this is the last kink to work out. It has 390 and 4 speed with 5 in lift and 35s so am really looking forward to driving it.

 

It sounds cool. Pictures please.