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Air in brake system

Hi, so I have a 97 Yukon. Had it sitting for a while before deciding to work on it and bring it back to life. Only issue I have left is the brakes. I replaced my master cylinder and bled my brakes. Front disc brakes are fine no air in them caliper pistons are still good no leaks. Now the back drums I have issues with. One of them constantly has air in it, I have one of those bleeder guns so you can bleed the system by yourself, with my rear breaks I can’t hold vacuum pressure with the self bleeder. I keep pumping it and it sucks air but can’t hold vacuum so obviously I’m thinking there’s a leak in the actual line or something but I’m not too sure and I’d like more opinions! Please help me out and tell me if you’re confused about anything I said I kind of suck at explaining things thanks!

If the cylinder checks out okay then maybe rusted out brake lines are the issue.

If you are sucking air with your vacuum gun and there is no brake fluid leaking anywhere I think the air is coming in the end of your hose where it firs over the end of the bleeder. You can try a dab of petroleum jelly over the bleeder or a very small hose clamp, zip tie, or twisted mechanics wire.

I have found that the jar type of one man bleeder works very well for me. You can buy one or just use a clear piece of plastic tubing that will fit tight over the bleeder and have a container fastened 6" or more higher than the bleeder with the hose going down into the container. Fill the master cylinder and pump your brake pedal 3 or 4 times. If you are anal about it like me, refill the m/c and do it again. Close the bleeder and done.

Great news, did some experiments! I have a worn out bleeder valve. I found this out by closing the valves completely and I was still able to suck out air with my bleeder gun when I should be sealed tight… hoping this solves my issues once I replace the valve!

My personal preference is not the vacuum brake bleeder method

My preference would be a diaphragm brake bleeder . . . but that costs some money

failing that, I would use the 2-man method. One man pumps and builds up pressure and holds the pedal down, at which point the other guy cracks open the bleeder valve. Google the procedure if you want a better explanation than I can give

I haven’t had good luck with those one-man brake bleeder gizmos,

Note these recommendations are meant for diy-repairs, because I got the impression the OP is a diyer

You can make a pressure brake bleeder for a little over $20 or you can get one already made that is similar for about $70 at Amazon. I got the one from Amazon and it works great.

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Gravity bleeding is so simple…you only need a piece of hose, a bucket and someone to push on the brake pedal.

Actually, gravity bleeding can also be accomplished with nobody pushing on the brake pedal, but you need plenty of time

I’m w/db4690, I’ve never had any luck with vacuum bleeding either. It might not be the valve itself leaking, it is probably seating air tight ok when closed in fact. It might be the connection from your vacuum pump to the valve has a leak. But more likely even if the valve seats air tight when closed, that doesn’t mean the threads will seal air tight when open, which it has to be when bleeding the brakes. In fact it would be surprising if the thread seal was air tight when the valve is screwed 1/4 turn open.

I presume you don’t have ABS, b/c bleeding w/ABS poses its own problems.

Suggest to try the gravity bleeding method. Just fill the reservoir, open a single bleeder, and let it drain. The MC is higher than the bleeder so the fluid has to bleed out eventually, and as it does it will push the air out of that line. Then repeat on the other 3. It’s a bit of chore b/c you have to keep an eye on how fast it is bleeding out so the reservoir doesn’t go dry. But otherwise it’s pretty simple to do for a one man diy’er job. Since you have a new MC, you don’t have to worry about burs damaging the seals, so you could hurry the process by combining gravity bleeding with the push on the brake pedal method too. Configure a stick w/the correct length it will hold the brake pedal 90% of the way down when jammed between the seat and the pedal. Open the valve, push on the brake pedal gently with your hand, prop it down with a stick, then close the valve, and release the brake pedal. Continue until you get fed up, then open the valve and continue by using gravity.

I disagree on both counts

Every 1997 Yukon sold in the US had ABS

And performing a brake bleeder procedure on this particular abs-equipped vehicle isn’t that big of a deal. It’s certainly possible to bleed the “base brake system” without a scanner, and even without any particular brake bleeder, and achieve acceptable results.

I should mention this is my right rear drum brake. Furthest from my master cylinder so one man method will be tricky. When the valve is closed I hold pressure in my self bleeder but I still suck constant air even with is sealed tight. When I crack it open of course it is more air being sucked out but this time no pressure as if I’m pulling air straight from a leak somewhere in the system.

Replacing the bleeder screw will hopefully be the solution.

What is a base brake system? Do you mean the ABS brake system that comes with the base model of the '97 Yukon?