Hey dudes!

So, I installed two MC/Boosters on my truck in the past month. The first one I got from rockauto and it was faulty so I went with one from Summit. Anywho.... The master cylinder keeps leaking from the cap seal (as pictured). When I park the truck and pop the hood after driving the sides and front of the MC are drenched dripping onto my steering box. The first one did the same thing but that's not the reason I returned it. The booster itself was bad. Any quick fixes for a problem like this? Thanks!

 

How full are you filling it???? Level should be 1/4"-3/8" from the top.....

 

Do the brakes work as they should? I have seen this with a bad MC plunger when one of the plungers is not sealing and squirting brake fluid into the reservoir at high pressure. I have also seen it when a proportioning/metering valve went bad causing alot of back pressure and blowing the MC plunger seals (Typically the rear brake plunger seal) or rupture the cylinder in drum brakes.

PS: Remove the MC cover and slowly!!!! push the brakes, if you see a high pressure jet of fluid rather than rolling turbulence in the reservoir check your MC plunger ("C" clip remove plunger and inspect) and then metering valve.

 

Mad of right now it's 1/2 full and still leaking when brakes are applied.

 

Hmmm.... that's good advice. My rear wheel cylinders are fine, I believe. No leaks, anyway. And it feels the brakes are acting as they should. My braking pressure is good at the pedal and it feels like front and rear are proportioned well. It brakes very well with this new MC/Booster (A1 Cardone).

The retainer bar that holds the cap down is pretty damn strong, as you'd expect, nothing out of the ordinary. Is surprising that anything is getting passed that seal and baffling at the same time. What's weird is the first one that had a bad booster did the same thing. Now my second is doing it. The stock one I replaced made the pedal go to the floor. Being 43 yr old truck, I decided to replaced both MC and Booster which I bought as a kit. Stock MC didn't leak at all. Maybe it could be a bad proportioning valve?

 

A good way to check the metering/proportioning valve is to go to a parking lot after it rains and have a friend watch wheel lockup. If the rear wheels are locking up first then you have a bad valve.

 

you should be able to get a new gasket at any auto parts store . clean the mating surfaces and replace that gasket.

 

I happened to replace my MC acouple of months ago. It hasn't been leaking like yours but the one I took off did the leak from the cover from the day I installed it over 20 years ago. Replacing the gasket made no difference. Turns out it would stay dry if I cleaned the gasket where it contacts the top of the resivoir. Wiping it down with a rag and making sure it was completely dry before installing it kept it from leaking, but if there was fluid on the gasket when I put it on it would leak till I cleaned it.

 

I tested that out in gravel. In reverse the rear would lock up first (it very well could have been all four). Forward the front would lock up first. I hope it's not the proportioning valve. I doubt I'll be able to get that off without braking lines. That **** is on there! Lol

 

i have a feeling this will be my best bet.

when I was bleeding the brakes after installing the booster; the pressure from the MC valves seemed normal.

 

Thats good to know. I'll make sure it super clean and dry (brake cleaner) and see if that does the trick before buying a new rubber gasket. This leak is quite annoying. Especially since both units leaked right out of the box.

 

On my 79 I replaced anything and everything that had to do with the braking system. Now I have the same problem with the MC leaking out of the top. Frustrating after putting that much money into it. Will try drying it also to see if that helps. Doesn't make sense since you have to about break your fingers to get the lid off/on. But heck it won't cost anything to try drying it first before installing it. Will give it a try.

 

I know it's strangew but that was my experience with it for many years. It was as if the presence of fluid allowed more fluid to wick past the gasket.

 

Rynoride,


Had the same experience you're having with the leaking master cylinder cap on my 77 F-150. I also purchased a new boot and it still leaked even with cleaning all the fluid off the rim and rubber boot. Then read in a previous thread someone else had the same problem and they resorted to using a thin piece of plastic underneath the spring clamps center "V" portion. I did that and mine hasn't leaked since performing this last July 2016. The thin piece of plastic I used was from automotive wire loom that I mashed together that seemed to be the perfect thickness needed to stop the leaking lid.


Within the previous post indicated manufactures / re-manufactures today have less quality control in their process compared to ~ 40 years ago and therefore have untrue (un-even) master cylinder opening surfaces. Hope this simple fix also works in your predicament.

 

A geyser of brake fluid shooting up out of the MC is and indication there's air in the brake system.

If the seals on either the primary or secondary MC internal pistons are bad, pressure will build for a moment (usually after pumping the brakes) but, will start losing pressure past the compensating port in the floor of the reservoir and the fluid will return into the MC reservoir (Internally bypassing), instead of the full hydraulic force being applied out at the brakes.

Assuming if all the air is out of the brake system and that there are no fluid leaks anywhere in the brake circuit, a simple test to determine if the MC is internally bypassing is to apply the brake pedal and just hold constant pressure on it for about 120 seconds. If the pedal remains at the same relative height off the floor, the MC piston seals are good. If during this time the pedal begins to slowly sink towards the floor, the piston seal(s)/MC bore are bad.

Basic operating principles of a brake MC.


If the MC/booster was recently changed out, the output rod of the booster should always be measured against the bore depth of the MC being installed (prior to mounting the MC to the booster), so that the booster's output rod is set to the correct length when the brakes aren't being applied. Too long of an adjustment on the output rod will make the brakes drag and the brakes will always be partially applied, even when the brake pedal isn't being applied. Too short of an adjustment on the booster output rod will cause the brake pedal to have an excessive amount of forward travel before any significant brake force begins to happen.

How to set the booster output rod length to the MC being installed.

https://www.ford-trucks.com/forums/1...l#post16825296