Hi,

So I've got an '81 F150 and the brake fluid in the front compartment of my master cylinder keeps disappearing. It is the metal container and the front compartment is much smaller than the rear compartment which does not seem to lose any fluid.

Does the front (smaller) compartment supply the front or rear brakes? I looked at my front calipers and rear wheel cylinders for leaks and there were none. So time to replace brake lines.

Do I now need to "bench bleed" my master cylinder? What ever that means?

Thanks!

 

The smaller front reservior is for the rear brakes. Disk brakes need more fluid because as they wear the piston moves out and needs the extra fluid to keep it full. Pull the drums to inspect the wheel cylinders for leaks, it may be small enough to not leak out a lot and be noticeable. To bench bleed a master cylinder you remove it (or usually before a new one is installed) and clamp it to a sturdy surface in the upright position. New master cylinders usually come with a bench bleeding kit, but it can be purchased separately. There are plastic fittings that screw into the ports and clear lines are attached and positioned in the reservior. The master cylinder is filled to the top submerging the ends of the clear lines. You then slowly push the master cylinder pushrod in, bleeding the air out. Tapping on the side of the master cylinder helps dislodge air bubbles. You're supposed to wait a minute between pumps but I'm never that patient and have had no problems. Keep doing this until all air has been expelled, you will see the air coming out of the plastic line, which should always be under the fluid so no air will be drawn in. When finished the plastic lines are removed and plastic caps are put on the plastic fittings, then the top is installed on the master cylinder. You can then bolt it on the truck and after hooking up the lines bleed the system. When you remove the plastic fittings with the caps to hook up the lines don't let all the fluid run out of the master cylinder, you will lose some but don't let it run dry. Bench bleeding is required with a new master cylinder because the brake pedal won't move the pushrod as far as you can with it removed. With that explanation out of the way I would say you probably don't have to remove it and bench bleed it, just bleed the brakes very well. If unsure about all the air being removed then bench bleed, but I've had partial brake failure many times and don't remove the master cylinder to bench bleed.

 

Wow, thanks so much for the detailed reply!

Cheers!

 

It does sound like bad wheel cylinders. If one leaks change them both. They are very cheap. If you wheel cylinders are leaking then you will also have to replace the brake shoes as well, as they will be contaminated with brake fluid. I will make the suggestion yet again is get yourself a pressure bleeder from this outfit http://store.motiveproducts.com/shar...unt2=344855904
you will not be sorry.

 

The hydraulic brake system is sealed and under thousands of psi when you push the brake pedal. If the brake fluid keeps going down in the reservoir there is a leak somewhere you just have to find it. Our buddy Just Call Me Sean has an awesome detailed explanation of bench bleeding but I agree with him that it may be a small leak you just have to find it. If all of your brake lines, fittings, calipers, wheel cylinders are dry (**check wheel cylinders by taking rear drums off**), then Im suspecting you have a bad master cylinder leaking fluid internally into the brake booster. If its not the wheel cylinders pull the master off and check inside the booster. If it is leaking get a new master cylinder and booster, and make sure you BENCH BLEED the NEW master just like Sean said.

 

I have to agree with Trans, the systems are separated, front and rears. This was done for safety reasons years ago. Fluid loss in forward most chamber will not get into the booster. Look for leaks in the back brake system, lines, fittings, wheel cylinders.

 

Yes Bill youre right, sorry I got myself slightly confused between front and rear while typing. ..................Anyway you said when you initially checked the front calipers and wheel cylinders, did you pull the drum off or just check the back side where the line screws in??

 

You should also check the rubber line going from the frame to the axle for wetness. Depending on your area you might have to check the entire brake hardlines from the master cylinder back, this is only needed if you live in an area that salts the roads during the winter.

 

the fact that your losing brake fluid should raise an alarm the part about there not being any tell tale signs is simply mistifying!i would replace both rear wheel cylinders and while your at it replace shoes and all new hardware giterdone!Possibly replace rear brakelines from wheel cylinder to block!!!dont forget to bleed rears first you would normally lose fluid from faulty whee cylinder this is an easy fix should take about one hour per side good uck

 

bench bleeding a new master is a must!