95 F250. Master cylinder or ABS pump?

[bbqjoe]

I have a sinking brake pedal.
I have also never had a Ford truck with outstanding brakes, but I believe these could be better.

I just replaced the brakes on the rear, including wheel cylinders.

Brakes bled, and adjusted, but still have a pedal that sinks, although the brakes are better than they were.
I did some research, and a bit of yootoobing, and it looks like the next things to troubleshoot are the ABS hydraulic unit, and the master cylinder.
Possibly the booster, but I'm tempted to believe it's working.

I've never dealt with a clear plastic master cylinder reservoir before.
Something I did notice when bleeding the brakes was that I could see fluid flowing upward in the reservoir at the rear of it when the pedal was pressed down and bleeders were closed. Not a geyser per se but definitely a good upward surge.
Is this normal behavior for a MC, or is it an indication of a bad MC?

If that's normal, I may have a look at the ABS unit, but for the price of a reman, I might just bypass that piece of junk.

 

[quaddriver]

it could be that the seal separating the high pressure front discs is leaking into the low pressure rears.

dont bother rebuilding it, just get a reputable reman.

do you have RABS or 4WAL? if just rabs the bleed is a single screw on the frame under the driver seat.

i you lost the booster, the brakes would get HARDER to push but would not sink.

personally I have always liked OBS truck brakes so something is amiss, you should be able to 4 wheel lockup in a panic.

 

[bbqjoe]

it could be that the seal separating the high pressure front discs is leaking into the low pressure rears.

dont bother rebuilding it, just get a reputable reman.

do you have RABS or 4WAL? if just rabs the bleed is a single screw on the frame under the driver seat.

i you lost the booster, the brakes would get HARDER to push but would not sink.

personally I have always liked OBS truck brakes so something is amiss, you should be able to 4 wheel lockup in a panic.

Thanks for your opinion!
I'm certain I could lock up the brakes if needed.
It's when going down a long incline, the brake pedal slowly goes to the floor.
It's easy enough to lift off or pump a little to start again.
But my wife might drive this occasionally, and I'd like to get it as close to normal as possible.

The MC isn't a big deal, nor really is the ABS pump. But my main question is if the MC is behaving normally.
Somehow it doesn't seem to me that it should be flowing back into itself upon pressing the pedal.
What do I know?
The MC I can get new, but I've heard nothing good about the ABS pumps and have read that it's better to go with an OEM reman on those, or just rig up a bypass.

 

[SoCal Sam]

First, make sure there is no air where the lines connect to the MC. With the pedal pushed down, simply crack the line and re-tighten. If that does not work, MC could have a bad seal. On a new to me car with spongy brakes, I replaced the MC and that took care of the issue.

 

[D_Zoot]

This sounds more like a master cylinder issue. If the ABS unit was leaking into one of the internal accumulators it usually results in a low pedal, but not one that goes to the floor. Typically an internal leak in the ABS unit affects only one channel on 4 wheel systems, and obviously only the rear on RWAL systems, leaving the rest of the system with full pressure. Also once the accumulator fills and the pressure equalizes the pedal stops sinking.

If the master cylinder cup seals are leaking it allows the pressure to bleed back into the reservoir and cause the pedal to sink steadily until it's on the floor. This may be the flow you are seeing back into the reservoir. Normally a little squirt back up on the very first movement is normal, but once the cup seal passes by the port it should stop.

 

[bbqjoe]

This sounds more like a master cylinder issue. If the ABS unit was leaking into one of the internal accumulators it usually results in a low pedal, but not one that goes to the floor. Typically an internal leak in the ABS unit affects only one channel on 4 wheel systems, and obviously only the rear on RWAL systems, leaving the rest of the system with full pressure. Also once the accumulator fills and the pressure equalizes the pedal stops sinking.

If the master cylinder cup seals are leaking it allows the pressure to bleed back into the reservoir and cause the pedal to sink steadily until it's on the floor. This may be the flow you are seeing back into the reservoir. Normally a little squirt back up on the very first movement is normal, but once the cup seal passes by the port it should stop.

Thank you.
I just picked up a MC.
Hope to get it installed tomorrow.

 

[gadget73]

agreed that sinking pedal sounds like an MC issue or a leak. I've had wheel cylinder seals and caliper seals fail and do basically this as well. Air in the ABS unit would make for a spongey pedal but it wouldn't sink. Also depending on rear ABS or 4 wheel ABS, it might only affect half the system. The proper way to bleed 4 wheel ABS is usually involving a scan tool, but I've faked it by forcing a lockup on dirt, then bleeding in the usual manner. A few cycles of that will get the air out of the ABS unit just fine, even if it does take a lot longer than if you had the proper scanner to force the bleeder mode on.

 

[bbqjoe]

agreed that sinking pedal sounds like an MC issue or a leak. I've had wheel cylinder seals and caliper seals fail and do basically this as well. Air in the ABS unit would make for a spongey pedal but it wouldn't sink. Also depending on rear ABS or 4 wheel ABS, it might only affect half the system. The proper way to bleed 4 wheel ABS is usually involving a scan tool, but I've faked it by forcing a lockup on dirt, then bleeding in the usual manner. A few cycles of that will get the air out of the ABS unit just fine, even if it does take a lot longer than if you had the proper scanner to force the bleeder mode on.

Thanks for your thoughts as well.
I'm going to replace the master cylinder, front brakes, go from there and see what I get.

Amazing place here.
I got way more input here than I did on two different Ford truck forums.

 

[NH-MAN]

Dont forget to bench bleed the master cly before installing

 

[gadget73]

I'd go so far as to bleed fresh fluid through the system while you're at it. If you're replacing the MC, thats half of the battle and it will need bleeding anyway. May as well just pump enough fluid through to get clear juice at the wheels.

 

[bbqjoe]

Whenever otherwise good people denigrate the bully breed, I think of images like this one.

View attachment 1596774


I've owned 3, and known many. To a dog, they were big, sweet goofy bear-traps. Terrible as guard dogs, since they love people.

All the contempt for the breed would be far better served to be aimed at the owners who put these loving animals on 3 feet of logging chain, away from the pack, and de-socialize them.

Dont forget to bench bleed the master cly before installing

I'd go so far as to bleed fresh fluid through the system while you're at it. If you're replacing the MC, thats half of the battle and it will need bleeding anyway. May as well just pump enough fluid through to get clear juice at the wheels.

Indeed!

 

[quaddriver]

ford OBS the easiest in the world to bleed off. just install mc, fill, and push till it bubbles at the ports then reconnect.

they will NOT gravity bleed, so do a gross bleed on all 4 corners in any order, bleed off the ABS unit - you are at best OBD 1.5 so a scan tool wont do anything for you, then bleed rear right, rear left, front right, front left and avoid hard stomps for any reason until all 4 corners are done or else you set off the lockout/brake pressure indicator.

the abs is pretty simple, red light = e brake on, or pressure low, yellow light = abs issues OR pressure low red on so pay attn to the lights on the dash

 

[bbqjoe]

Front brakes done, rear brakes done, MC done.
Entire system bled.
Found a major chunk of air in the ABS valve.

Brakes are way better than the day I brought it home.

Thank you for all your input gents!

 

[SoCal Sam]

Nice job. My Crown Vic had spongy brakes when bought. When I got a hair and decided to add traction control, I had to change the MC, ABS valve block, ABS module and a couple of the hard lines. IIRC, bleeding the valve block was done with a diagnostic tool which opened the valves so that fluid could flow. Don't have the tool so I had to trigger ABS and re-bleed. Still spongy. There was air in the lines feeding the MC. That took care of it. Today, some dufus cut me off on the freeway forcing me to apply enough brakes to trigger ABS. All good.