Cold Brakes

jeep
brakes
cherokee

#1

The brake pedal is extremely soft when the car is parked in cold weather. Ther eis still brake function but the pedal goes mostly to the floor. As the temperature rises, normal pedal function returns. The car is a 2001 with 130K on it. I have had the brakes looked at by numerous shops and no one can determine what is going on.


#2

Do they get harder when you pump them?

I’d suspect air in the lines. Air has a far higher coefficient of expansion for temperature changes, and air in the system would effectively lower the volume in the lines as the temperature drops. It should also get harder when pumped.


#3

They do pump up a bit but not until the temperature warms up. I have had air in the lines investigated–fluid had been drained and refilled 3 times, no leaks, no sign of air.


#4

It might be your master cylinder needs a rebuild. The seals might work poorly when cold/stiff, better when warm/pliable.


#5

seconded.


#6

yep, an o-ring seal in the master cylinder is allowing by-pass leakage, (internal).


#7

I have had that checked but will investigate further


#8

Actually, now that I think of it, I replaced the master cylinder and no change


#9

On my truck, I had to replace the master cylinder on the clutch three times before I got one that didn’t leak. And they were all factory replacement parts, not third party or re-builts.


#10

I have thought of that and agree that is probably the next thing to do–thanks


#11

Wow… this is a tough one. I’m a brake engineer and this is a bit of a stumper.

I don’t think it’s air or the master cylinder… with the fluid warming up and fixing the problem. Brake fluid in the whole system stays pretty cold when the air temp is cold. There’s really nothing to warm up the brake fluid. Down at the caliper on a LONG drag, the brake fluid may get warmed up and even boil in a poorly designed caliper… but up stream… again not much there to warm it up.

You didn’t say if the car has ABS or not. There is an accumulator for the ABS system that may have a warm/cold issue.

Have you been monitoring the brake fluid level in the reservoir? Are you losing fluid?


#12

It does have ABS and the brake fluid level stays full. This one has stumped everyone who has looked at it.


#13

This is a long shot, but…
Have you checked for bad wheel bearing? Maybe you have a marginal one that tightens up when it warms up. A very slightly wobbling rotor will spread the pads apart and cause the long pedal throw that you describe.


#14

Sometimes long shots are right on–in fact, the rear rotors do wobble and need replacing. I have had the front wheel bearing replaced but not the rear.


#15

Is it cold to hot that tightens up the pedal, or that you’ve pumped them and closed up clearances? In other words, do you ever get a long pedal when its warm? Can you give any more detail to what you’re experiencing?


#16

How about a sticking caliper? Not the piston stuck in the bore, but a caliper that won’t slide?


#17

Never have a soft pedal in warmer temperatures–it seems to happen around 32 F and below. When they are soft and cold, pumping helps to restore pedal function but it realy does not return to full pedal until the ambient temperature warms up.