I'm at a loss here. After I drive my F250 for a short ways (5-15 miles) the master cylinder wants to leak out of the seal between the cap and the cast body. The fluid level is about 3/4" down from the top edge of the MC. I have replaced the rubber seal and it still leaks. I'm running Sanderson ceramic coated headers - could this be the culprit - too much heat? The body of the MC is ~ 140 degrees F. as read with a laser thermometer after these short drives. Any suggestions? BTW: this is a new MC, brake booster, all new wheel pistons/ cylinders and brake lines and new fluid, the whole brake system is new.

 

How good is the sealing surface? I remember a couple years ago on some Mustang forums it became a big problem with new and reman m/c having rough castings and crappy rubber gaskets.

Otherwise, excessive pressure in the reservoirs would come from a frozen wheel cylinder or piston.

Josh

 

Is your master cylinder lid vented? (there's a small vent punched into the original MC lids), i went through the exact same scenario you're describing when i replaced my MC last summer, to make a long story short i replaced the non-vented lid that came on the new MC with my old vented lid and problem solved, it had me stumped for awhile but the MC has been topped off for over a year now without anymore leaks.

 

Bullitt - the top edge of the MC is a bit rough (looks like pitted metal). Both gaskets are new. One has smooth sealing edges and the other has small ridges that run the perimeter of the sealing surface. What should I do about the rough casting? Both are REMAN's and are identical in the casting roughness.

Montana - both MC came with unvented caps. Where do I get a vented cap because I don't have my old MC. Can I drill a small hole in the cap somewhere?

Thanks for the suggestions guys!

 

You can mighty file the sealing surface.

Interesting about the vented versus non-vented caps.

I would probably re-bleed the entire brake system too just to make sure.

Josh

 

I suppose drilling a tiny vent hole would serve the same function, i would just pull a vented lid from the bone yard and give that a try....the rubber gasket between lid and reservoir works like a diaphragm or "bellows" in that it is set up to move with the fluid that seals the system and it's the vent in the lid that allows the diaphragm to move, without the vent a vacuum is created between lid and gasket forcing fluid out from under the lid.

The issue Bullitt raised is an all too common problem as well, if the casting is really rough i'd suggest removing the MC and working it back and forth over some emory cloth placed on a flat surface until you get a nice smooth sealing edge.

 

guys, i checked my lid and it is vented already. It must be the rough casting causing the problem. I think I'm going to take some JB weld and fill in all the pitted rough areas, sand it nice and smooth, re-bleed the brakes and cross my fingers it doesn't leak anymore. Really sucks to have brake fluid leak all over a newly painted engine compartment and frame....damn it!

 

Same problem here. I adjusted the holding clip which holds the cap to the main body and problem solved.

 

Well the JB weld did not work. Instead, I sanded the top of the MC body on a belt sander. This took away all the pitted parts of the casting and is as smooth as glass now. Re-bled the whole brake system, rubber cap seal is new. If this doesn't work, I'll be p...ed off. Fingers crossed there won't be any leaks next time I drive it.

 

Are you using one of those generic flat seals? or the original style seal with the collapsible "bellows"?

 

are you using synthetic fluid? mine began leaking after topping it off with some synthetic i had laying around. after flushing with non-synth, no more leak.

 

montana - original bellows type seal, new with the MC

solafide - DOT type 3 brake fluid, non synthetic

My other concern is excess heat in the engine compartment from exhaust headers. I have Sanderson ceramic coated headers. Not sure if the fluid is getting warm enough to expand in the MC or not from the exhaust heat. The fluid level is down about 3/4" to 1" below the top of the MC. The MC's temp the other day was 140 degrees after a drive which is far from boiling or excessive heat that brakes are subjected too. I'm driving the truck again tomorrow to a truck show, we'll see what happens on this trip.

 

The boiling point if brake fluid is 500+ degrees and there a lot more confined engine compartments that have no issue with overheated brake fluid.

Josh

 

Like Bullitt said, boiling the brake fluid isn't your issue here, keep us posted...

 

Wow, I need to stop posting with an iPhone... Autocorrect and grammar is atrocious!

Joah