In the past few weeks, I've noticed that my brake pedal is quite a bit softer first thing in the morning vs. after I've driven it, say, 15-20 minutes. Once I have a few miles on it, the pedal is much firmer and the pedal travel decreases also.

At first, I just kinda thought my groggy head in the mornings was playing tricks on me. But I'm now convinced this is a real phenomenon.

I thoroughly flushed/replaced the fluid about 6 months ago, the pads at all 4 corners are in good shape, and I have a new master cylinder about 4 months old. I've noticed this effect in ambient temps of 0-10'F and also up to 60'F.

If there was a problem with any part of the braking system, I would think just the opposite would be true here -- the brake pedal would become softer after a few miles. But maybe I'm way off here.

Anyone have any ideas? What gives?

 

I bet the caliper slide pins are beginning to stick - could be any one of them too...

What happens is the caliper won't slide, so the outboard pad isn't really being pressed against the outer side of the rotor. The caliper pushes the inboard pad against the rotor, which may or may not flex enough to meet up with the outboard pad. After you get some heat in them, the slide pins start to move again.

Grease 'em up

On the other hand, the ABS unit might actually be leaking slightly internally - either way, take it out in a dirt lot or even an empty parking lot, and slam on the brakes hard enough to get the ABS going - that'll free up the slide pins, and make sure the fluid in the ABS unit gets circulated ...

 

Ah yes, I'm familiar with the calipers hanging up on these trucks. So if the slide pins are sticking, would they bust loose if I stand on the brake pedal really hard?

 

Not necessarily

I've "been there, done that, got the t-shirt" - if the bins are dry, the more pressure you put on them, the more they bind up it seems.

The ABS "judder" seems to help loosen them up better than anything else I've tried. Besides, of course, taking the calipers off and freeing up and greasing the pins

 

What's the generally accepted/recommended grease to use on the slide pins? I think the thinner grease (lower viscosity) the better to enable them to slide smoothly? White lithium OK? I'm doing them this afternoon or evening....

 

High-temp, high-pressure, I would think.

 

The slide pins looked OK. There was grease on them - not too much - and grease was in good shape. Wasn't dried out and seemed to be moving alright. But the calipers and rotors are noticeably hot after a 20 mile drive. Infrared temp readings on the front rotors was about 160. Rear rotors was 250. I wonder if the pistons within the calipers are hanging up. They're original from the factory I'm sure.