pedal sank to floor under constant pressure. replaced master cylinder. now pedal goes to floor with a clunk/thunk sound. The booster rod extends out 0.995 inch so it's not too short. pedal movement of an inch results in movement of the fluid in the front reservoir of the master cylinder. It's a 1984 F250 (8600 GVWR, 351W, 4x4). any ideas on tracking the issue would be greatly appreciated.

Welcome to FTF, Monk_1! I moved your post to the appropriate forum so that the right folks with answers might be more likely to see it.

Pardon my ignorance but does this truck have disc brakes in the front and drum brakes in the back, or are they drum brakes all the way around?

Ryan

it has disc fr, drum (12"x3") rear. just figured out how to get back to you. took me a minute.

O'Rattlecan said:

Pardon my ignorance but does this truck have disc brakes in the front and drum brakes in the back or are they drum brakes all the way around?

Click to expand...

1968/72 F250 2WD: Drum brakes were standard equipment, power front disc brakes were optional.

1973 F250 2WD: Power front disc brakes became standard equipment.

1976 F250 4WD: Power front disc brakes became standard equipment.

No rear disc brakes in F250's until the mid 1990's.

If there is no hydraulic pressure on the brake pedal then the replacement MC is toast

They are not always good right out of the box.The sealing rings don't always do their job

I suggest you get an MC with a stainless steel bore.

T

Was the replacement master cylinder "bench bled" before it was installed?

Was the entire brake system including the proportioning valve bled?

If the answer is NO to any of the above, the problem...most likely...has nothig to do with a faulty master cylinder.

With the engine off i have pressure to half-pedal, where it thunks and sinks to the floor. With the motor running it pushes straight to the floor with a thunk at half-pedal. I bench bled the MC, and bled the system but for a proportioning valve. Didn't know about that. Have to look in my Chilton's for it's location and bleeding procedure. i've read it through several times and recall no illustrations or directions for bleeding the proportioning valve. Suggestions?

The proportioning valve just directs fluid to the brakes pistons it does not bled seperately

If all 4 wheels are bled to free air, thats got it.

It also sounds like the BB has a vacuum problem.

Have you inspected the check valve ?

Not yet. Just came off a double. In looking through my Chilton's it reads (in relation to bleeding the system) "It will be necessary to centralize the pressure differential valve after a brake system failure has been corrected and the hydraulic system has been bled." I've never encountered the term "centralize" as it's used here. Nor have I worked with a 'pressure differential valve'. Is this in the body of the MC? I know you don't bleed a proportioning 'valve', but I was assuming that the person who said I need do this was, in fact, referring to the PDV. Where is this valve, and what is 'centralizing'? I'd like to thank you for the time you've taken to reply to my questions. My truck is in pieces and I'm using the wife's mini-van til I get this figured out. The sooner the better. Does the 'boot' that the BB rod project out of go bad? Is this how it fails? Is this rod attached inside the body of the BB? Is it supposed to slide in and out freely?

So I figured out the PDV and the centralizing of same. Would still like to know more about the BB. When I removed the MC the BB rod slid out and air filled the vacuum through the boot the rod projects out of. Wondering if the rod is supposed to freely move in and out, and whether the vacuum loss at that boot indicates a bad BB.

Brake boosters can go bad

Right now I only have instructions and illustrations for the 70's trucks

They will be similiar though.

If the Brake failure light on the dash comes on with the key ans stays on, there has been a hydraulic line failure. If the brake failure light goes out after a few seconds you DON't Have to centralize the rod in the PPV

I replaced the master cylinder on my 1983 F250 last month.The BB rubber seal that the rod slides in and out has to be in good condition. It seals vacuum. If you wiggle the rod side to side very gently, you should hear air escaping. Do Not remove the rod !!! Adjustment is at the tip with a lock nut if you need to adjust.Is the vacuum hose from the manifold in good condition, and the grommet on the BB (top left corner) There should be slight friction when the rod slides in and out on the seal. Hope this helps. BTW, you could have a defective M/C. Happened to me when I got a SH-T rebuild from that other country that seems to be making everything now. Spend the extra and got one from the good old U.S.A. Enough said:guns

Great information. If possible I'd like a copy of Figure 7, as well of Parts 12-50/52/58 (instructions on a dash-mounted booster pushrod adjustment). I can figure a fix, now. I've done this once or twice and never encountered these problems. Most of the work has been on older rigs. First 80's rig I've owned. Never cared for the body style. It's a good rig but for being old and previously owned by the indifferent. I'm working toward a brand new truck one part at a time. Thanks

Excellent. Thank you.