I noticed the master cylinder was empty from sitting (few months), I put some fluid in and it just poured out the main seal. I got a new master cylinder and bench bleed it until there was no air bubbles.

I installed it and when I went to bleed the rear first, one side was fine and moved to the driver's rear. Fluid was coming out just fine, but all the sudden fluid shot out the top of the reservoir (like a fountain). Then I figured I would try the fronts but no fluid would come out at all at the bleeders.

The pedal has a little play but feels pretty good. In the manual it says to release the differential valve pin, but I can't figure out how to pop it out. Also I read that if you pull the pin, the valve will start leaking and I don't need to release it to bleed the brakes.

It's a 1975 F250 with dual piston front calipers and power brakes. The fluid shooting out the reservoir really freaked me out, I never have had that happen. It seems like something isn't letting it go toward the calipers and pushing out the reservoir. Trying to get this back on the road, any help is appreciated.

 

Did you have the cap on the reservoir while bleeding the brakes?

 

No cap was off.

 

Cap should be on...unless there is a new trick? Make sure you bleed a little at a time and keep the reservoir full. Start furthest away from the m/s.

And are you holding/blocking the proportional valve pin in or out based of GVW? I think on a F250 it "out".

I use the clear tube on the bleeder plug and in a clear container trick.

 

It sounds like he tripped the check valve in the proportioning valve, andit is stuck. You are going to have to play with the valve and try and free it so it will center. I haven't had that problem in years, so I can't give you any advise on freeing it up, maybe someone else here has played with one recently.

 

I hear that ck valve can get stuck and the way to un-stick it, is to stomp on the brakes......with the lid on.???

I have not heard of bleeding the brakes with the m/s lid off?? How would you build up any pressure? Seems to me you would just push out all the fluid right out the open top??

 

Cap should be off to allow any air bubbles to escape, plus it's easier to top off the fluid while bleeding. Pressure is from the master cylinder pushing to the lines, it should not allow pressure to go back up to the reservoir. It's a one way system. I've bleed brakes probably 50 times on numerous brands and always left the cap off.

I think you are right about the check valve, but in the picture it shows holding the pin out but I can't get it out. Is this because the check valve is stuck? Quite a few threads I read said to not mess with the pin while bleeding because pulling it will cause it to start leaking and I would have to get a new valve. They said you can bleed the brakes normally without pulling the pin.

It's a F250 Camper Special 8000GVW with dual piston front calipers by the way. Thanks for the quick replies.

 

Whether the cap is on or off doesn't matter when bleeding the brakes. The cap doesn't hold any pressure. The reservoir is just that - a reservoir. I typically leave the cap off just because it's easier to watch the fluid level.

1975FordF250 is spot on, although air bubbles will make their way to the top with the cap on or off. However, you shouldn't have air bubbles going this direction if the MC is already properly bench bled. Air bubbles should only be going down toward the wheels and out the bleeders.

The proportioning valve pin MUST be held OUT while bleeding the fronts. Read, MUST. On some heavy duty trucks, it's pushed IN. Note that this mechanism is independent of the pressure differential valve. The BRAKE light should come on if the pressure differential valve is off center. The BRAKE light should also come on as the key is turned to START via the PROVE-OUT.

 

Okay, brake light doesn't come on at all. I will check the connection again. I guess I will try to pull the pin again, should it only be pulled out slightly (1/8") like in the picture or more? So the pin has nothing to do with the pressure differential valve, it's just a bypass for the fronts? Thank you.

 

Pull the pin out as far as it will go. It doesn't go very far. You're correct; it's independent of the pressure differential valve and acts purely as a bypass so that fluid (and air) can move straight out to the bleeders without having to first push against the metering spring, which could compress and trap the air you set out to remove.

As a built-in safety feature, the brake light cannot come on during PROVE-OUT if the switch is disconnected.

 

Okay, the pin on mine looks like the end is worn down or something because I can barely grab it with a razor blade. I just have to figure out a way to hold it, I might just super glue a metal rod onto it and try that. It needs to be pulled out the whole time I bleed the front brakes, and closed when I do the rear brakes or does it matter?

Also I assume the brake light switch/sensor is suppose to be on the proportioning valve? I just see a plug on mine and the connector hanging there. I don't see a sensor anywhere else. Maybe some one took the sensor out and put a plug in? Thank you.

 

The position of the pin doesn't matter when bleeding the rear brakes because the two circuits are independent of one another. Just release it before the vehicle is driven.

It's tough to convey what "a little play" is over the web, but if you already have a solid pedal, chances are you're not going to gain anything by repeating the procedure with the pin managed properly. You can get lucky and not need to use it at all; in fact many folks don't even know about it. However, the proper procedure does in fact require the use of the pin.

 

By a little play I mean maybe 20% of the pedal travel until it feels hard, not perfect but not horrible. I will just bleed the fronts and hope for the best.

 

If the pin needs to be held out, make a sheet metal V the metal thickness/distance to hold the pin out.

 

the valve has 2 functions, if you lose pressure on either the front or rear,(brocken line, or leaky cylinder), the piston moves and shuts off the flow to that section so you will still have the other 2 brakes. What happens is, if you bleed them slowly, it wont shut off the other system, but if you're too fast and hard wwith the pedal it thinks it has a leak and shuts off the other side. At that point you have to manually reset it to the middle if it's sticky or you wont have the other set, in his case while he was bleeding the rears, it shut off the fronts..