Master cylinder/Hydro booster

[brndnreb]

Hey fellas. My first time to post here. Have had an 2013 lml for years and now final getting to point where I have the time/space to work on my truck myself. It used to be my primary driver but not any more so I can take my time fixing it. Anyway; thats the quick background. On to my question:

I have had a power steering leak for a while and have gotten around to tackling it. The leak was high and near the back of the engine compartment so I figured hydro booster. I've replaced the hydro booster which required me to remove the master cylinder and disconnect brake lines to get it out. That required me to bleed the brakes. The truck has 255K miles and I don't remember if the brake fluid has ever been changed so I decided to flush the whole system while I was at it.

Once finished I took it for a test drive and the front brakes locked up. I bled the bleeder screws on the front calipers and that released them but they are still locking up and I have to relieve the pressure every couple of miles.

I'm thinking I stirred some crud up (the old fluid was pretty nasty) and blocked the return hole in the master cylinder so I'm thinking replacing that is next. I'd be interested if you guys think I'm on the right path.

Also, when looking up the part, and the master cylinder comes back with a couple different part numbers NM4214, NM4215, and NM4216. It seems like the difference is stability control and brake controller. I have both stability control and a brake controller (truck's a denali model). It seems like the 4215 is what I need but before I buy it; I'd like to get a 2nd opinion. Thanks in advance guys.

 

[Ron Nielson]

Use your VIN# to do the parts lookup on a GM-based website and get the correct part number. With the correct part # in hand, you can search and decide where yu want to buy your part, OEM or aftermarket, etc.

Replacing the master cylinder and flushing the entire braking system, fronts and rears - clean it all out and replace with new fluid. Good idea. Your pressure may be there because you have air in the braking system that heats up and supplies pressure to the calipers.Once all the air is gone, you should have great brakes again.

 

[brndnreb]

Use your VIN# to do the parts lookup on a GM-based website and get the correct part number. With the correct part # in hand, you can search and decide where yu want to buy your part, OEM or aftermarket, etc.

Replacing the master cylinder and flushing the entire braking system, fronts and rears - clean it all out and replace with new fluid. Good idea. Your pressure may be there because you have air in the braking system that heats up and supplies pressure to the calipers.Once all the air is gone, you should have great brakes again.

So would you recommend bleeding them again before replacing the master cylinder? I guess its certainly possible there is still air in the system. But I would have guessed that that would have made the brake pedal softer than normal as opposed to harder.

 

[Ron Nielson]

Since you removed the brake lines exiting the master, I would loosen each line independently and bleed the master to make sure all air is out of the master, then do a full 4 wheel brake bleed. If your brakes worked before you replaced the booster, they should work after the booster replacement. You shouldn't need to replace the master at all.