Stuck Master Cylinder Cap

Hey all, I've been working towards starting my truck, and the brake pedal goes straight to the floor. But suspiciously, the master cylinder and lines all look relatively new compared to the rest of the truck.

I wanted to check the master for fluid as a step 1, thing is I cannot get that plug screw out from the top of the reservoir! I don't want to throw any heat on it for fear of melting some gasket and/or rubber parts. I've been blasting it with kroil for a few days, but it just wont budge.

Am I missing some kind of tab or lock? Or is there any advice anyone has to get this thing off?

Thanks and Merry Christmas!


  • LanceLance Posts: 882Member
    What you are dealing with is a nasty combo of water and brake fluid.  It is a tough bond when it's on a cap. I would use   vice grips and the kroil you already have. A bit of elbow grease should get it.   Try and tap it with a hammer and then try to turn it. The new master cylinders use plastic caps and don't have the problem like the old origional caps did.   Good luck.
  • lostmindlostmind Posts: 1,274Expert Adviser
    If you have an air hammer you can use it straight down on cap , the vibration should loosen it. Chance are you will find it's corroded inside.
    They're pretty easy to remove , half an hour job. You can work on it on the bench and inspect the bore.

    Make it safe.
  • 27esx33t827esx33t8 Posts: 14Member
    If you have an aluminum master cylinder, most likely it will be severely corroded, best to get a new cylinder.
  • longlivelancelonglivelance Posts: 80Member
    Thanks for the info everyone. I’m going to pull it and look at it on the bench. Sounds like chances are I’ll be getting a new one for it anyways. 
  • longlivelancelonglivelance Posts: 80Member
    Just couldn’t get this thing off. Finally just removed the master cylinder engine o try and get it off. Upon removing the master cylinder it looked like someone may not have screwed the cap all the way shut and as a consequence it rusted shut. After a lot of cranking to try and get it loose it finally just snapped. That thing was never coming out.

    Guess that answers my question about needing to take care of that. 😂

  • GeoffGeoff Posts: 3,867Senior Contributor
    New ones are not that expensive, and easily available. 

  • Uncle JoshUncle Josh Posts: 2,646Senior Contributor
    NAPA 4360
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