Losing brake fluid

Hi,

The booster won't cause it to lose fluid. Sometimes, a bad master cylinder will leak out the rear and into the booster, but that isn't the booster causing it.

I assume you checked wheel cylinders and brake calipers for leaks. If there are absolutely no leaks, the only thing that can be happening is the front brakes are wearing quickly and causing the caliper piston to be extended further. The extension requires more brake fluid to remain in the caliper.

Also, check around the reservoir for evidence of a slight leak. Also, remove the rear wheels and brake drums. Pop open the rubber boot on each side of the wheel cylinders to see if they are wet. See pic 1 below.

Let me know.

Joe

The rear seal at the back of the master cylinder can leak brake fluid into the power booster where it will be sucked into the engine and burned, but that isn't horribly common. You would have seen wetness there on the old one. To still have such a slow leak, the best suspect is a leaking rear wheel cylinder. When it's so slow, it's going to drip onto the drums, then be burned off or mix with normal brake dust to form a black sludge. You may never see wetness on the inside sidewall of the tire or even on the drum.

To verify this, you'd have to remove the drum, then peel both boots back on the wheel cylinder and look for excessive wetness around the pistons.

As recently as the 1980s, we used to rebuild wheel cylinders as part of a standard drum brake job, but today new wheel cylinders barely cost more than the kits to rebuild them. If you need to replace one, check out this article for more information:

https://www.2carpros.com/articles/how-to-replace-a-wheel-cylinder

Also, if it will help, here's a dandy video on bleeding the new wheel cylinder:

https://youtu.be/w7gUsj2us0U

This isn't important right now since you have a new master cylinder, but for the benefit of others who might come across this while researching a similar problem, if you resort to pedal-bleeding with a helper, never push the brake pedal more than half-way to the floor. Gunk and corrosion build up in the lower halves of the bores where the pistons don't normally travel. When you push the pedal over half-way with a master cylinder that's more than about a year old, the rubber lip seals run over the rough stuff and they can be ripped. That results in a slowly-sinking brake pedal, and that often doesn't show up for a few days.

I only use gravity-bleeding where I wait for the brake fluid to run out of the bleeder screw, then I close it lightly. Next, I irritate the brake pedal a little by hand to wash any trapped air bubbles into the wheel cylinder, then I open the bleeder screw once more for a few seconds to let them out.

Be aware you'll cause a lot more trouble from needing excessive bleeding if you let the reservoir run empty while the wheel cylinder is unbolted from the steel line, (or if the hydraulic system is opened anywhere else). A simple trick to prevent the fluid from draining out is to use an appropriate stick from the seat to hold the brake pedal down about one or two inches. That will move the pistons forward in the master cylinder just enough for the lip seals to block the fluid from leaving the reservoir. The only air you'll need to bleed out will be what's in the new wheel cylinder and maybe a few inches of that steel line.

Hi Joe. Happy Friday to you. Looks like great minds think alike.

Hi,

You are very welcome. Also, I was wondering if any progress was made?

Let me know.

Joe