88 Bronco II. 2.9L, 5spd.

OK, drive around town for a bit. All is well. Drive on the highway, and the front brakes start getting hot and locking up.

The rig has been sitting for quite a while, so I'm wondering if the return springs in the calipers are weak? If there's too much rotor contact with the pads since they're not retracted enough..

Could this be the issue? Is there something else I should be looking at?

-Steve

 

These disc calipers don't use any springs. Where it has sat for a while, I would probably look at the slide pins that hold the caliper in place and allow it to slide. With age and lack of use, they may be a little sticky/rusty not allowing the caliper to slide back. The other thing I would check would be to "bleed" the front brakes to make sure that brake fluid flows freely to the caliper.

 

Well, let's be clear. Calipers do not use any type of "return spring". Rather they use a very well thought out design to apply the brake pads, and return them to their normal position. If you remove the caliper? You'll find a rubber dust boot covering the outer edge of the piston. Just below that? Is a square cut rubber seal.

How this all works? Simple. You apply the brakes, the piston comes out....the rubber seal DISTORTS slightly outwards at this time. Sense the caliper is mounted on either pins or a slide? It acts like a huge vise clamp. Piston comes out--inner pad contacts rotor....The caliper itself slides slightly (on the pins or slides) back using that piston pressure against the inner pad against the rotor, thus moves the outer pad into contact with the rotor.

Now when you let off the brake? All that pressure comes off. The seal that was slightly distorted 'outwards' will actually pull itself back a touch, along with the piston. The caliper is 'released' in that state, and the whole "clamp" relaxes..taking also the pressure off the outer pad.

So what can be causing your problems? Well...Sounds like you are describing a classic dragging brake problem.

This is caused normally by several different things. a) Fluid level in the master cylinder to HIGH! If someone overfilled it last time it was checked? That will wind up holding some pressure in the system. b) The slides/pins that the caliper rides on? And this is true if the truck has not been driven in a while? Probably need to be cleaned up and given a fresh coat of caliper "lube". Over time dust/rust/crud can build up on the pins/slides and prevent the outer pads "clamp" action from releasing.

Or c) Internal seal on the caliper is worn to the point it can't "deflect" back into the home position correctly, and gets either sticky---Or (you'll love this) It CAN actually lock up the brake! and not release it. Had that one happen on a friends car a while back. It was spewing smoke from the wheel well area---the rotor was severely over-heated and warped.

Suggestion would be to first check fluid level, to ensure the master cylinder is not "to the brim" with fluid. Then I'd pull the calipers, and clean the snot out of the slides/pin contact areas with a wire wheel. Same for the slides/pins themselves...Clean 'em fully. Reassemble with caliper grease. (DO NOT USE STANDARD CHASSIS GREASE!!) I personally use either a graphite lube, or Silica-Glide caliper lube. (ask around. Lots of decent caliper lube products available which might work better for this job)

Give that a shot and see if it helps? If it still acts up? Might need to replace the caliper(s). (rule of thumb...when one caliper craps? other side is not far behind). Ohhh side bar note. This can and most likely will also cause your rotors to warp. Don't mean to be a pain in the rear end--but if you get it freed up, but you find the brake pedal starts to move 'up & down' while braking? Most likely the rotors will need to be either machined or replaced (most of the time? Replacement--anything newer then mid 80's used very thin cut rotors right off the line...)

There are of course, other rare/oddball things that could also be the cause. However you'll probably want to start with the easier ones first.

Best of Luck with it!!

S-

 

(almost forgot. Thank You for your service to your country!!!)

S-

 

Cody's Bronco

The calipers are cheap enough that I think I'm going to pick up a set on the way home from work today. Pads too. Just to be comfortable knowing that my 16 year old will have mostly new front brakes. I'm hoping that our test ride yesterday didn't heat the rotors too much to be warped.

If this isn't it.. (master cyl fluid level is fine), then could this be the power booster?

-Steve

Oh, and thanx. But there's millions more following in my foot steps. Please encourage others to support them too.

 

(always do--to answer the last question)

Very unlikely the power booster, unless it has been recently changed out and the pushrod was adjusted incorrectly.

The basic test for a power booster is quite simple. With the truck 'off' depress the brake several times to deplete the stored vacuum. You should feel the pedal getting high and very stiff. Hold the brake pedal!!...and start the engine. The pedal should drop 1-2 inches roughly. (used to be part of our NYS inspection, haven't worked as an inspector in 16 years LOL.. But the operation should be exactly the same.

S-

 

So far. I took it for a good abusive test drive and it seems fine now. We haven't driven this rig much. Son was just so intent on driving it to the first day of school tomorrow. He got his license just a couple weeks ago. The thing runs like a top. We're the second owner. The original lady put 115k miles on it as her daily comuter.

Thanx for your comments.

-Steve

Now he wants a 2" lift. Power windows and locks. I think I'll look for an Explorer radiator as I've flushed this one and changed the thermostat and it still tends to run a bit warmer than I'd like it to be. Crimony, I made the mistake and told him that a 5.0 will fit! Damn it. I just happen to have one in pieces out in the shop.