Brakes trying to lock up

[Chilimac]

I have a 2005 Chevy Silverado and a couple of weeks ago I was driving on the highway on cruise control at 78 mph at about 2100 rpm's and suddenly my rpm's went up to 4000 so I cut off the cruise control and then the front end started shaking so I pulled over and I could smell my front brakes. I inspected the truck and couldn't find anything wrong with it so I proceeded to return home but my brake pedal was very hard. It was obvious that the brakes were trying to lock up but I don't know why. I'm being told the brake booster/master cylinder, so far I have done nothing to the truck, want to get some advice 1st if you could please help I would appreciate it very much.

[Jsdirt]

No idea why the truck would suddenly whip up to 4k - usually that's a sign of some serious transmission trouble completely unrelated to the brake issues.

 

Calipers on post '96 trucks have become so craptastic that this has become a regular occurrence on all of them. Flushing the brake system every few years is now mandatory if you want them to last. What probably happened is your brake fluid has been in there since the truck was new, turned to crap & is loaded with contaminants, and has stuck one of the pistons in the calipers.

 

Back in the day you could just flush the fluid out, push the piston all the way back in, then pump the brake until it's just slightly farther out than it was, & push it back in again, then remount the caliper, pump up your brakes and your done. Sometimes I'd even pop the piston right out, clean up the seals and the seal grooves, and reassemble. I wouldn't dare try that with these new ones. They don't even sell rebuild kits anymore since they've become cheap throwaways. These days it seems it's very difficult to save a caliper from the scrap heap. I've done 7 of them the past 2 weeks! Plus over 30' of brake line ....

 

While a master cylinder problem is certainly a possibility, my money is on a bad caliper, since it happens so often these days

 

 

Then there's the lovely steel bleeder screws in an aluminum caliper body, with ZERO anti-seize compound ...

[Doug_Scott]

There are only a couple of common reasons for the brakes to stay applied. First and most common is as Jsdirt posted, you have a caliper starting to seize. Keep in mind that the only thing to pull that caliper piston back from the pad is the square cut o'ring inside the caliper. When the piston gets pushed against the pad, the "o'ring" will twist slightly. When brakes are released, the o'ring returns to original shape, pulling the piston back slightly. If the o'ring cannot twist, the piston will stay against the pad.

 

Then next most common reason for a caliper to stay applied is the actual slides that the caliper is supposed to be able to move freely on. If they seize, they may stay applied.

 

There are other possibilities, but, they are rare to occur. There is a compensating port in the master cylinder reservoir that allows brake fluid to vent back into the reservoir once pedal is fully released. This port can become plugged with crap in the brake fluid. Once you get to thinking it may be the booster, you are really reaching for it. There is also the extremely rare issue of the flex hoses starting to come apart inside the actual line and having the lining start to come apart and turn into "flapper valves" that can stop the return of fluid to master. It is a possibility that is taught in trade schools, but, I have never actually found it to happen.

 

Keep in mind that once the caliper starts to stick, it will start to get hot, that heat gets absorbed by the brake fluid, causing the fluid to expand, and if the compensating port is blocked, the hot fluid will expand enough to apply the brakes more, causing the engine to downshift because you are applying more throttle without really noticing. You do notice the downshift though.

 

Brake fluid is hygroscopic. That means it absorbs and holds moisture. The fluid will darken with the moisture, and after a few years, it can contains so much water that it will boil under normal braking conditions. Changing the fluid on a regular basis will help deal with that.

 

All of the above happens on every disc/drum brake vehicle out there, adding in anti-lock brakes adds in lots more places to screw up. I left the trade before anti-lock became normal, so cannot offer any possibilities from that part of the system. Generally though, start with the basics. More often than not, most issues have nothing to do with added electronic.

[Jsdirt]

HA! I didn't even think of a downshift.

 

Been a long day trying desperately to cobble a '00 Silverado back together again ...