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turbojimmy's avatar
turbojimmy
Explorer
Oct 04, 2014

Hydroboost Master Cylinder or other Leak?

So I was giving the RV the usual visual checks before I left for a short trip yesterday. I saw that there was a disturbing amount of fluid on the bottom of the hydroboost pump. I ASSumed it was PS fluid, since the high pressure hose was wet. I wiped it down, started it up and worked the brakes and steering to see how much fluid would re-appear. None did.

I drove the campground yesterday (10 miles) and the brake pedal started to get spongy. Parked for the night. When leaving this morning the BRAKE light came on as I was pulling up to the dump station at the campground. Pedal went almost the whole way to the floor before braking occurred. I had every fluid possible in the storage compartment except for brake fluid.

My dad swung by with some fluid and scooped up my kids. I didn't want to drive home in an 8-ton RV with questionable brakes with them in it.

The front reservoir was dry; rear was full. I looked everywhere for signs of a leak on the front lines. There weren't any, but the bottom of the hydroboost pump was covered in fluid again. Now I'm pretty sure it was brake fluid. Particularly since the power steering fluid level has not changed.

Usually when a MC fails, it's the plunger at the pedal, which in this case would be on the Hydroboost side of the MC. BUT, usually both reservoirs (front and rear) drain, right?

Any guesses? MC? Just the cap?
Class A

4 Replies

  • turbojimmy's avatar
    turbojimmy
    Explorer
    Oct 05, 2014
    Update: It's the MC. It's clearly leaking between the reservoir and the base. I hope the lines come out!!
  • turbojimmy's avatar
    turbojimmy
    Explorer
    Oct 05, 2014
    Thanks both for the tips.

    I can't tell what color the fluid is because there's 30 years of grime over in that corner. It's kind of a brown color by the time it drips off the bottom of the master cylinder. It's thin, though, like brake fluid. PS fluid is a similar color but thicker.

    The inside of the left tire has streaks from some sort of fluid, but I don't think it's the caliper or line over there. The fluid is not on the rim - I think it's fluid blowing back from whatever the leak is. There's a substantial trans fluid leak as well that I plan to seal up while I'm in there. It's just the rubber hoses on the after market trans cooler - easy fix.

    Anyhow, I ordered up a new master cylinder. It has rear disc brakes so the reservoirs are the same size. It can't hurt to replace it anyway - assuming the lines come off. Got a PS hose, too, though I'm not 100% sure that's leaking now.

    Now that the rain has stopped I'm going to pull both front wheels and make sure there's no other leaks.
  • spacedoutbob's avatar
    spacedoutbob
    Explorer
    Oct 04, 2014
    Master Cylinders are designed to split the front from the rear braking systems, If it shows that it is wet where the master cylinder is mounted, it is probably leaking from the master cylinder itself. You can check to see if the power braking system is working, by pumping the brake pedal with the engine off until the pedal is hard, then start the vehicle, the pedal should drop, if it remains hard, then you have a problem with the power booster. I'm more inclined to say it is your master cylinder. The larger reservoir in the master cylinder is for the front disc brakes, the smaller reservoir is for the rear drum brakes.

    Good luck,

    Bob in Calif.
  • Golden_HVAC's avatar
    Golden_HVAC
    Explorer
    Oct 04, 2014
    Hi,

    The brake fluid is nearly clear, and will harm the hands when it is on the skin. Brake fluid also takes off paint. The power steering fluid might be yellow or more likely red.

    Chances are that you will need to have the brake master cylinder replaced. The brake booster seems fine at this point.

    Fred.

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