The forward compartment on my master cyl seems to be getting pressurized to the point of blowing fluid out the cap. I have bent the holddown spring and even smeared a tiny bit of gasket sealer on the gasket to prevent this to no avail. I've bench bled the master, bled the brakes,and adjusted the drums all the way around. The light from my pressure differential switch is on and won't go off. Something is screwed up and I have no idea exactly what. My brakes seem ok but that forward compartment gets completely drained of fluid and and don't see anywhere underneath where it's leaking except out the cap. I need some help. I drive this thing 60 miles a day. My only running rig at the moment. Thanks Oh yea It's a 69 f250

 

The reservoir closest to the radiator supplies brake fluid/pressure to the rear brake circuit.

If the brake warning light on the dash is lit up, it means there's a differential in pressures between the primary (front) and secondary (rear) brake circuits.

Most likely, you're only running on the front brakes and the rear brakes are dead.

You didn't mention if this is the existing MC you bench bled or, if it's a remanufactured/new MC you've installed (?). There was also no mention if your brakes are manual or power assisted.

IF there's no brake fluid leaking out of the wheel cylinders or calipers (if you have front discs), hard lines, flexible brake hoses or at any of the brake system fitting connections this pretty much only leaves the MC as the problem.

IF you have manual brakes, look on the inside of the firewall (behind the padding) to see if brake fluid is running down on the inside of the truck. IF you have a brake booster, loosen the two nuts holding the MC to the booster (will require a 9/16" socket), slide the MC forward on the booster studs and see if the back end of the MC is wet with brake fluid.

Depending whether or not you have manual or power brakes and if you find either of these scenarios to be present, you've found the source of the leak. If you have manual brakes, get a new (NOT rebuilt) MC. If you have power brakes, you'll need a new MC and a newly rebuilt booster --brake fluid will attack the rubber diaphragm of a vacuum brake booster and cause it to fail, if brake fluid has come in contact with it.

Once you've located the source of the problem and get it corrected, you'll have to recenter the pressure differential valve spool inside the brake valve. With the MC bench bled, installed and the MC plumbed up and filled with brake fluid, open a bleeder on the primary side of the brake system (front brakes). Turn the ignition key on to illuminate the brake warning light. Slowly press down on the brake pedal to begin building pressure on the rear brakes. When the pressure on the rear brakes is sufficient enough, it will force the differential valve spool back over to the center. The instant the warning light goes off, get off the brakes, close the front bleeder and turn the key off.

You can continue on finishing bleeding out the brake system, starting with the rear brakes and working your way to the fronts. RR, LR, RF, LF.

 

The brakes are manual, the fluid is definitely coming out the cap as I have wiped it down ,gone for a drive, and the outside of the MC is wet again. It is the existing MC. I got a new MC from LMC truck but it is not the right one. I pulled the MC out and took it apart. All the cups and o rings are in good shape as is the inside of the cyl. It actually looks pretty new inside, not all grunged up as some I have seen. Especialy for a 47 yr old truck. I will try centering the press div valve and see if that helps. I,ve been researching this a bit on this forum and it seems there is a tool to keep the press dif valve from moving when bleeding the brakes. Any one know where i can get one or how to make one?? It seems bleeding the brakes would create a pressure differential and cause the valve to move again.

 

Your only rig is a 50 year old pickup, and you have a 60 mile daily commute? Yeesh.

Find something else to drive for a while till the brake system gets rebuilt. The brake pedal itself is probably OK but, everything else is suspect till proven otherwise.

 

Seems odd, if the fluid is just running out from under the MC cap, that it would only leak from the secondary side of the MC and not both the primary and secondary sides.

The pressure differential valve lock tool you mention is available from MuscleCarResearch.com .

This is the valve lock tool from them that I have.

 

If you get everything buttoned up and still have problems with the MC leaking (wherever the fluid may be escaping from), this new MC should solve your problems, if you have all-wheel manual drums.

BrakeBest Brakes Select NMC1386 - Master Cylinder | O'Reilly Auto Parts

 

When yer jeep tranny pukes and your riding your motorcycle in the Pacific Northwest rainy cold winter and ya work 6 days a week, you suddenly get very unparticular about what you drive. It was close by and when I punched it, it didn't blow blue smoke. Good enough. oh yea the heater worked. And it does stop. Nifty little tool. Interestin that the master cylinder pictured looks exactly like the one LMC truck sent me that didn't fit right and had too short of a stroke.

 

Did you use the pushrod that came with the purchased MC or, did you swap in the original pushrod from your truck, --if there's a difference in pushrod lengths between the two?

 

used the pushrod that came with the new one. It didn't fit the firewall well either. Too close. The wire that holds the cap on hit a bolt and would not allow removal of the cap. And the little trick to center the press dif valve failed to turn the light off. I'm gonna take it apart and physically center it. And inspect it.

 

Well OK. But, there's a certain element of choice involved here. Unless you live in the Land Time Forgot there are plenty of trucks a hell of a lot newer than 1969 to drive. Jus' Sayin'.

 

How long has the brake light been on? If it's been on since you got it then you'll have to remove it clean the crap that freezes the sliding rod from moving.

Then center the rod & insert your tool and only remove after you've bled the brake system. Bing no light if system has no leaks.

Orich

 

Since you mentioned the forward bowl (secondary side) of the MC had run out of fluid, it just seemed logical that the PDV would have shifted to the secondary side and that you would need to open a bleeder on the primary side to recenter the valve spool.

Since it wasn't mentioned if the brake warning light has or hasn't been lit up since you've owned the truck, it's possible that the valve spool is shifted to the primary side and that you would need to open a bleeder on the secondary side to recenter the valve.

Then again, it could just be gummed up and won't move.

Muscle Car Research also offers a reseal kit for the drum/drum PDV.

 

IF the replacement MC's pushrod was shorter than your existing MC pushrod, the old pushrod would needed to have been swapped into the new MC.

It may be that the wire bail on the replacement MC could have been taken off, turned around 180° and reinstalled on the replacement MC, for it to clear the head of the mounting bolt on the firewall for the pedal support that it was hitting on.

It's not uncommon for people to say they have a problem with brake fluid leaking out from under the MC lid. As I said before though, I find it really unusual that your MC only leaks fluid from the secondary side and that it has run ALL the fluid out of just that bowl and not both bowls. There would had to have been some pretty extreme braking or hard cornering to have sloshed ALL the fluid out and to have run the secondary reservoir completely dry.

Fluid may be leaking out from the lid but, my suspicion on it is still that there's also brake fluid leaking out from the bore at the rear of the MC and into the inside of the cab and is probably the most likely reason the reservoir ran completely dry. That, or, there's brake fluid leaking out somewhere else in the system that you just haven't spotted yet.

 

Pulled the PDV apart today and it was stuck in position with the rear brakes blocked. Got that cleaned up and back together. I made that nifty little tool with a piece of flared brake line ,a flare nut, and a piece of i think 8 gauge copper wire. I will put the whole mess back together tomorrow and see what happens after I bleed the brakes. As far as changing the push rod Really??? The stroke and bore of the master cyl are what determines the volume of fluid going to the wheel cylinder. The push rod is designed to give the proper stroke in the proper application. Change it and you compromise the operation of the brakes ,possibly to your great detriment. Just why would any intelligent wrench want to do that??? Why would the manufacturer send something like that without explicit instructions concerning such a thing??? It would open them up to HUGE liabilities. Really??????

 

I had my "new" MC Chinese junk leaking from the lid as it had a redesigned air lid vent.
It had been moved to the front & rear corners of each fluid wells at the one side of each of the lid.
It was a just a round raised area that looked about like a 16 gage wire could lay into those two areas. I tried flattening both areas down that slowed it down.

Rebent the wire bail but it was made out of some softer metal so used my old one.
I found a new chrome one at the good guy car show for like $8bucks that looked more like the oem style with the vent in the upper part of the lid.

The new chrome MC lid was the fix to my fluid dripping lid problem was now fixed.
Orich
1. was from retainer lid wire bail was to soft of temper