Brakes randomly sticking.

[Pbtrilogy]

Hello,

On my way home from work this past saturday, I started smelling burning brake smell and found that it was coming from the front passenger side brake. I drove it a bit on Sunday and had no issues. Today, on my way to work I started feeling the same grabbing feeling and smelled the same burning smell and saw smoke. But this time it was on the driver side and the passenger side was fine. Now I am afraid to drive it home and am not sure what it could be since it's not consistent.
Any ideas of what could be wrong or what I need to do? I am pretty new to these trucks and have had this one since October with no issues.
Thanks

 

[1987 GMC Jimmy]

Do you feel it pulling to the left or right when you brake? I would pull the wheels and inspect the calipers because it sounds like they're sticking at varying times. They also may not be returning sometimes when you release the brake. Sometimes you can clean up debris or rust and get them working again, but it may be time for new front calipers. Not terribly expensive. If the wheels don't turn jacked up, or they do with high resistance, that's a pretty good sign. While you're there, make sure your pads and rotors are alright. Also if your caliper bolts are in bad shape, get new ones.

 

[Pbtrilogy]

Well I decided to slowly and easily drive it home while partially using the parking brake. I did not really notice any pulling to either side. I looked at the brake fluid and the fluid is very dark with what looks like some silt looking material in it. The driver side did warm up but not nearly as hot as this morning.

 

[GTME94]

The flexible brake lines can also go bad and not release the pressure from the caliper. usually not random but it could happen. If they're old its probably good insurance to change them.

 

[1987 GMC Jimmy]

Well, let me say this, too. It sounds like your brake system is extremely contaminated, and you need to flush your brakes. Badly. Get something to draw as much fluid out as possible like a turkey baster, wipe the reservoir out, fill it up with new fluid, and you need to go bleed all that black stuff out of there until you start bleeding out fluid that's the color of new brake fluid. The bleeder valves on the rears are on the back face of the wheels, and the front bleeder valves are on the calipers. It would be easier to take the front wheels off, but you can do all of them with all the wheels off or on. Start on the wheel furthest away from the master cylinder and work your way to the closest one. You can open the bleeder valve, get a buddy to pump the brakes down and hold while you collect the ejected fluid, close the bleeder valve, have them let go of the brakes, and repeat. That's the two person method. The one person method would be going to say Harbor Freight tools, buying a bleeder kit, and doing it yourself per its instructions. Just don't let the master cylinder go dry on fluid while you're bleeding the brakes, or you'll have to start all over again because you don't want air getting sucked in there. I would very highly consider replacing your calipers and almost as urgently suggest for the master cylinder, as well. I don't know if you're having trouble with your rears, but new wheel cylinders may be a good idea. I don't give it the same urgency because it's farther away from the contamination, but if it's been like that for a while, they might be damaged, too. I know that sounds excessive, but you can't be too safe when it comes to the braking system on a car, and contamination is a potentially very destructive on brake parts depending on how long it's been like that. I'd say a good grocery list is two new calipers, a master cylinder, two new wheel cylinders, and enough DOT 3 brake fluid to do the job. Pads are cheap if you want to do those, too, while you're down there. Again, I know that's a brash suggestion for me to make, but safety is more ensured, and you would only have to deal with this once for a very long time, especially if you flush your brakes every 25-30K miles.

 

[Pbtrilogy]

Looks like I got my next day off planned out for working on it. I appreciate the info. Im guessing it would probably be a good idea to replace the lines too? If I do replace the lines, should I plan on replacing them with longer lines if I plan on going with a 4 inch lift in the near future?
Thanks!

 

[1987 GMC Jimmy]

Like he said above, the flexible ones are more vulnerable, but if they look okay, I'd hold onto them. Unless the steel molded lines are rusting out and leaking, there's no need to replace them. You'll potentially need longer flexible lines if you lift, but I don't know the specifics necessary to facilitate a four inch lift.

 

[chengny]

The fact that you experienced a non-release condition in both calipers - within a short period of time - is an important symptom to take note of. It suggests that, rather than a problem with a component associated with one side (i.e. a hose or caliper), there is an issue with something common to the entire front braking circuit.

It could be related to the PCV (proportioning combination valve), but that would be highly unlikely. It is more likely to be master cylinder related.

Can I make a suggestion?

Before you remove the brake fluid from the MC, do the simple test below. It will take about 2 minutes and will check for a blocked compensating (vent) port in the master-cylinder - possibly caused by particulates in the brake fluid. It will let you know if - in addition to flushing your brake fluid - you will also need to change the MC.

Upon release of the brake pedal, the compensating port(s) is uncovered by the piston as it retracts in the cylinder. Normally this opens the port and allows the pressure that is trapped in the associated lines and cylinders to be released back to the MC.

If the port is obstructed/restricted, backflow to the reservoir is slowed or blocked. This condition can be caused by contaminants in the brake fluid, corrosion of the cylinder or port, or a pushrod which has been adjusted too tightly.

The test:

Remove the MC cover, have someone push gently on the brake pedal and then release it. Watch the reservoirs and see if a little geyser bubbles up within each. If you don't have 2 geysers, one of the MC pistons isn't returning to its proper resting place. FYI - the rear chamber is the reservoir for the primary (front) hydraulic circuit.

From GM:

With the master-cylinder lid off, slowly depress and release the brake pedal several times. As you are releasing the pedal a steady stream of brake fluid should be visible bubbling up from the bottom of the master-cylinder. If no stream is visible siphon or remove all brake fluid from the master-cylinder and verify that the compensating port is not blocked.


The master cylinder consists of two pistons that work in tandem to apply pressure to the braking system. These pistons have cup seals that seal fluid in the pressure chamber. As the seal moves past the vent or "compensating" port it begins to create pressure in the pressure chamber through the lines and onto the brake units.

The replenishing port located next to the vent port allows fluid flow to the low pressure side of the piston. As the piston travels through the bore, vacuum is created behind the piston. This flow prevents any vacuum pressure from holding back the piston as it travels forward. A return spring forces the piston back to its resting position as the pedal is released. This action allows the brake fluid to return from the brake lines and piston chamber into the reservoir.

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If the brake pedal is over-adjusted or there is debris blocking the vent port, pressurized fluid will not be able to return to the reservoir. This can result in residual pressure in the braking system. This residual pressure will cause brake drag and lock up. Loosening the flare nut at the master cylinder port or at the brake units will relieve this excess pressure, but will not fix the underlying cause.

 

[Pbtrilogy]

Just did this test, and I have a stream/geyser from front and back when pushing the brake in, but not when releasing the brake

 

[chengny]

IDK - to be honest, I have never paid much attention to when the geysers occur (during release or application).

But if you have severely contaminated brake fluid:

I looked at the brake fluid and the fluid is very dark with what looks like some silt looking material in it.

especially the silt part, it may be best (time and money wise) to just pull the master and take the bleeder screws right out. Use compressed air to blow the contaminated fluid out of the lines.

Then when the lines are purged, buy a new master cylinder. Bench bleed/install it and fill the system with fresh brake fluid. Refit the bleeder screws and bleed the brakes as normal.

Changing a MC and flushing/bleeding the lines is much easier than doing calipers, wheel cylinders and flex hoses. And, I'm sure it would be especially annoying, if you did change all that and the front brakes still didn't release.