Brake master cylinder leak

[MaverickZ71]

2009 Silverado crew cab, 5.3L LC9 engine, 6L80E transmission.  

 

Brake fluid keeps mysteriously slowly disappearing from master cylinder.  (Several small bottles of DOT3 over the last couple of years.)  When master cylinder gets half full, MIL lamp lights and Service Brakes Soon appears in DIC.  Brakes function normally, even when fluid level gets lower.

 

Local dealer says they THINK (?!) the master cylinder is leaking into the brake booster [they say to diagnose will require removal of master cylinder and brake booster and by then I'll be on the hook for a few hundred $], requiring replacement of both the master cylinder and the brake booster.  Does that sound right, that the brake booster would also need replaced?  Same dealer says I can't do it myself, because I would have no way to properly bleed the ABS brake system.  Is that correct?  

 

I've driven many vehicles a couple of million miles, but I've never had a master cylinder leak before.  Drum brake wheel cylinders, yes, but not a master cylinder.  

 

Thanks!

[Jsdirt]

Yeah, that's a common issue. Since it's disappearing with no leaks anywhere, the only place it can go is out the back of the master cylinder and either down the front of the booster, or into the booster. 

 

The reasoning for replacing the booster is the brake fluid severely degrades the rubber diaphragm in the booster. It makes it swell up and wrinkle, so it will take more vacuum to do the same amount of work. Not only that, but you know how they build stuff today. You could probably drive a truck from the 50's-80's, and early 90's for years with brake fluid in the booster without even knowing, but these days it'll reveal itself pretty quick.

 

You can do that yourself. They're full of crap. I've replaced them before without even bleeding the system after, although that's not recommended. When you disconnect the system that high up, alot of the time you can purge the air out with several very light, short pedal applications with a several minute break in between - you'll get the air bubbles to rise right out of the master. Real small bubbles that resemble foam in a way - you'll see them in the fluid. By bleeding, you're drawing the air DOWN. Much easier for air to go UP. You'll definitely need to bench bleed the new master, though before installation. I do it in the vise.

Edited January 14, 2021 by Jsdirt

[Doug_Scott]

Just unbolt the master cylinder from the booster and pull the master cylinder forward enough that you can either see behind it, or can get a finger behind the master to see if it is wet inside the booster.  The only times I have ever had to bleed brakes at the wheels is when there was a part replaced after one of the flex hoses, or there was a complete brake fluid change to get rid of the water laden brake fluid.

 

Bench bleed the master before changing it, and leave the bleeder lines on (assuming they still supply a bleeder kit with master cylinders these days). 

 

Bleeding at the master after installing it is not done the same as the bleeding at the wheels.  To bleed the master after installing it, do not pump the pedal, just loosen the lines, have someone press the brake pedal down and then hold the pedal down as you close off the lines.  Once closed, wait 10 seconds and have the helper release the pedal.  Wait another 10 seconds, then crack the lines loose about a half turn and have the helper push the pedal down slowly, making sure they don't pump the pedal.  Close the lines as the fluid comes out, and have the helper release the pedal after you have tightened the lines.  It should only require a couple of bleeds to get a good pedal.

[Jsdirt]

^^^ That's the correct way to do it. My way is because I'm always flying solo here, lol. Would be great to have an extra set of hands in this place!