So I've searched this forum extensively, but haven't really found anything that matches my issue. This will be long winded as I attempt to describe the issue and what I think I've figured out.

So I have a 2002 GMC Jimmy SLS that I just bought used. It has almost 290,000 km. When I bought it first thing I did was change all the fluids. Engine oil was changed regularly but everything came out pretty black. The tyranny fluid was a little more brown than red and had a slight burned smell, but nothing me or my buddy found concerning. He used to work for GM so he has some experience with these. I did a drain and fill with a filter change for the transmission. I also changed the plugs, even that one bugger behind the steering shaft. I have put almost 10,000 km on it since than.

About a month ago I was at a red light and when I attempted to go it was hard to get moving, like I was towing a load. After some driving on the highway to get home it sorted itself out. I have a scanner with live data that let's me see transmission temperature. Normally it gets to about 71°C, but this time it got up to 93°C. When it stopped acting up the temp started to drop again. I installed a Hayden cooler with thermal bypass inline with the stock cooler.

At first I thought torque converter issue or pressure control solenoid after looking through this forum. Fluid level is good in the transmission, live data didn't show anything abnormal except the temp when it acts up. It doesn't do it all the time, but it does happen frequently.

I than noticed the brake pedal is extremely hard to push when this happens and it shakes like the brakes are sticking. No brake dust on the rims or the smell of burning brakes, but lots of heat comes off all the brakes when I stopped to check. You could feel the heat radiating off of them.

I don't think it's a stuck caliper as it's not just one wheel getting hot or pulling to one side. The pedal does pulsate when I step on it when acting up, but not in a way where I think it's would be a hydraulic issue, more of the brakes are just hot when I step on them.

I've tested the booster by stepping on the pedal with the engine off, holding down pedal and starting the engine. Pedal goes down like it should once it gets vacuum. The line also seems fine and doesn't run like it has a vacuum leak. When I took the line off and started it lots of vacuum through the line.

I think it's a brake booster issue, but like I said, I don't have a lack of brakes. When it's acting up and the pedal is hard it's very easy to stop as I think the brakes are already partially engaged.

Any suggestions? Like I said, feels like I'm towing a trailer, doesn't always happen, motor doesn't run weird, hard brake pedal but no loss of stopping power.

 

A few thoughts come to mind.

Do you have good vacuum? This powers the brake booster so perhaps that is the problem? I'm pretty sure the vacuum hose to the brake booster is separate from the rest of the vacuum lines under the hood but perhaps there is a leak in the hose for the booster or the smaller hoses that is limiting the vacuum that is applied at the booster.

Stuck/sticking caliper: I wouldn't discount a stuck caliper or piston. I learned the hard-way that ignoring the lubrication of the guide-pins can cause dragging brakes. You could have this on both sides at the front or back or perhaps even just one side in the rear. There is also the possibility of a stuck piston in the calipers. I would think a visual and mechanical inspection of these is called for. Doing this is not hard to do.

That you have some pulsation could be evidence for the brakes.

 

It does have good vacuum. When I checked with the engine running I didn't measure it but it was strong. I'll check the calipers next, it's just weird that it's intermittent

 

Pull the cap off the master cylinder and make sure:
A) It's not overfull.
B) The rubber diaphragm isn't swollen (which would indicate that somebody put a petroleum-based fluid in the brake system).

When it's acting up, jack it up and see if you can turn a front wheel. Or stop on a slope, put it in neutral, and see if it will roll on its own. That will tell you if the brakes are dragging.

If the transmission were starting out in second gear, it might cause similar symptoms when taking off from a stop, but it wouldn't make the brakes get hot.

 

So I just went for a trip today pulling the boat. Everything was fine up to the last 5 minutes when the issue happened again. I spent the afternoon on the lake, hoping the down time would let the issue stop as it sometimes does.

On the drive back it happened again, could barely make it up a hill. I pulled over on a slight decline. Vehicle wouldnt move. I then unplugged the vacuum line, plugged it back in, issue disappeared the rest of the trip.

Vehicle doesn't start in second. brake fluid isnt overfilled.

 

I use an IR thermometer to check my brakes every now and then. It's easy to walk around the vehicle and shoot the temps and compare them. They are all different but fairly close in temp. If any are off, I know right away where the problem lies. Is it possible that the problem is your cat?

 

So you tow a boat? I do too. I lost my rear brakes because the rear guide-pins got corroded and the caliper was stuck. Also, I learned the hard way to never tow anything with any sort of weight using overdrive. It cost me a transmission rebuild.

That the problem appeared to go away after you removed the vacuum to the booster (I'm assuming the booster) is a strong clue that it is not calipers. If you can "fix" it again by removing the vacuum hose, then that could be pretty good proof that the booster is to blame. Unfortunately, I don't know the failure mechanisms for those things so I don't know how that would cause the symptoms.

 

I'll check the temps when I go for a drive next. I really think it's the booster and start there unless the temps show otherwise. I definitely tow in 3rd, fuel is cheaper than a tyranny, even with these fuel prices. I'll keep posting results in case anyone else gets the same issue. Just waiting for parts to arrive.

 

So an update. It acted up again and I took the temperatures. All 4 were hot and not just one. So I figured I either have incredibly bad luck or it's not a sticking caliper issue. I have just replaced the booster and have gone for one test drive and the issue hasn't happened again so fingers crossed it's fixed.

I will need to replace the master cylinder as there was some brake fluid at the back of it where it connects to the booster.

I am also debating on doing a full brake change including calipers. They've definitely been heated up. And experience has shown me that that dragging brakes cooks calipers and they tend to fail shortly thereafter. Powerstop sells a decently priced kit through Rock Auto.

The booster replacement was less than pleasant. The one bolt took forever as it turned 1/16 of a turn at a time. Getting the booster out of the engine bay was problematic as well. The whole process probably took me 3 1/2 hours.

If the problem persists ill update this post.

 

Thanks for the update!