Merkur XR4Ti - Figuring out a soft brake pedal

This one has me scratching my head.... Subject car is a Merkur XR4Ti with rear disc brakes. Brakes were working fine despite the fact that I had stopped half-way through a change of rear calipers, running a T-Bird caliper on one side and a Lincoln LS caliper on the other. I actually had been running like this for two years without issue - the calipers were pretty much the same size, so no biggie.

Jump forward to a week ago, I finally decided to get the LS caliper on the other side. All went well, bled just that corner initially, went to go for a test drive - woah, way soft pedal! Bled that corner some more as well as the three others, a little better, but still way soft. I can get a harder pedal by pumping it up some, but obviously that's not right.

I took it out for a test drive yesterday, really stomping on the pedal a few times, to the point of lock-up. Oddly enough, the only corner that is locking up is the left-rear, the one where I replaced the caliper. Aren't I correct that if there was air in this corner, it would be LESS likely to lock up?

I'm at a loss as to what to do next. Thoughts?

Brad

Have you double checked the new caliper for sticky pistons/leaky seals ? That's all I can think of.

P.S. I'll be trying out your "10,0000 post" front brake upgrade in the near future.

Did you make sure the bleeding screws were seating correctly? If the brake fluid was old and cruddy, you could have a bleeder not seating correctly--gunk in fluid or as tincetti alluded to, gunk/crud in the "new" caliper preventing proper seating.OR

Did any part of the master brake cylinder or "circuit of the braking system" drain completely? If so, you still got air in the lines.

Rebleed everything. Wipe down all the junctions/connections, and they lay Kleenex--one layer--on them. If there's a leak, you'll find it.

Really sounds like you have air in one of the brake circuits.

triumph5 wrote: Did you make sure the bleeding screws were seating correctly? If the brake fluid was old and cruddy, you could have a bleeder not seating correctly--gunk in fluid or as tincetti alluded to, gunk/crud in the "new" caliper preventing proper seating.OR Did any part of the master brake cylinder or "circuit of the braking system" drain completely? If so, you still got air in the lines. Rebleed everything. Wipe down all the junctions/connections, and they lay Kleenex--one layer--on them. If there's a leak, you'll find it. Really sounds like you have air in one of the brake circuits.

x2, we've had soft pedal on stock setup, little weeping from one of the drums, rebleed it and voila, fine, just try again, get a second person to pump while you make sure everythings ok

Have you ziptied the brake hoses?

Take a bundle of heavy duty zipties and install them on the hoses about 1/8" apart, rebleed the brakes one more time then drive again. If it suddenly is awesome you have at least one weak hose. I suggest replacing all of them with a braided Goodrich type hose.

triumph5 wrote: Did you make sure the bleeding screws were seating correctly? If the brake fluid was old and cruddy, you could have a bleeder not seating correctly--gunk in fluid or as tincetti alluded to, gunk/crud in the "new" caliper preventing proper seating.

I'm getting fluid through the bleeder, and it's now very clear. Seems to all be functioning as you'd expect. I should add, I needed to screw the piston in a bit to retract it, so it's certainly not seized. The pins are lubricated and the caliper slides easily on them.

triumph5 wrote: Did any part of the master brake cylinder or "circuit of the braking system" drain completely? If so, you still got air in the lines.

I made sure the reservoir always had enough fluid. When I changed the caliper, it wasn't open very long at all - a couple drops of fluid were lost.

Any thoughts on the fact that the changed caliper, the left-rear, is the one that locks up first under hard breaking? I sorta expect the rears to lock first with this setup as currently configured, but what is it telling us that one side is readily locking ahead of the other?

John Brown wrote: I suggest replacing all of them with a braided Goodrich type hose.

I've already got SS braided hose at all four corners in good shape.

Do the rear calipers have to be adjusted manually ?

tincetti wrote: P.S. I'll be trying out your "10,0000 post" front brake upgrade in the near future.

Just make sure the caliper donor is a 2008+ Focus. Might have to buy new - I was just lucky to find one in the junkyard that they put out right away since it was a train wreck.

DPDISXR4Ti wrote: Any thoughts on the fact that the changed caliper, the left-rear, is the one that locks up first under hard breaking? I sorta expect the rears to lock first with this setup as currently configured, but what is it telling us that one side is readily locking ahead of the other?

1: A hose is expanding and dispelling hydraulic pressure.
2: The new caliper has a different internal diameter.
3: Your master cylinder has failed (happens often with XR4Ti brake services if the calipers are "opened" without opening the bleeders.)
4: There is air loss on the other circuit.

My ziplock trick helps diag line #1.

It's telling you there's uneven pressure to the rears. Are the calipers from the same make and model, or, and I have no idea, is it possible that they vary slightly in bore or internal size? That may account for the pressure imbalance.

Or maybe the new one is operating as it should, and the other side needs the attention!

iceracer wrote: Do the rear calipers have to be adjusted manually ?

Generally, no. They're a relatively modern caliper, as used on the ~1999 - 2004 Lincoln LS. However, you raise a possible point... I had to re-work the e-brake cable and it's currently loose as a result - I need to get under the car and tighten up using the center adjuster. I wonder if there could be something going on in that regard?

triumph5 wrote: Are the calipers from the same make and model, or, and I have no idea, is it possible that they vary slightly in bore or internal size? That may account for the pressure imbalance.

At this point, the calipers that are now on there are the exact pair that was pulled off the low-mileage junkyard LS. Ironically, previously there was the somewhat mis-matched pair on the car, and they worked fine.

It just strikes me as odd that if there's air in the system somewhere, it's almost assuredly in the left-rear (where the new caliper is installed), and yet that's the one that is locking up first.

It's working properly, and the right is hanging up internally or externally.

ADD: You still, though, have air someplace in the system. Rebleed.

check to make sure the brake pad is seated correctly

triumph5 wrote: ADD: You still, though, have air someplace in the system. Rebleed.

Indeed I did. The problem was that even though the bleeder screw was at the highest point, there still must have been some air pockets captured in the caliper. By unbolting the caliper, rotating it so that it was vertical, the air pockets must have then rolled out and I was then able to fully bleed with nothing but fluid inside.

Rock solid pedal now - all good!

tincetti wrote: Have you double checked the new caliper for sticky pistons/leaky seals ? That's all I can think of. P.S. I'll be trying out your "10,0000 post" front brake upgrade in the near future.

Have you tried it yet?