Had my MC re-sleeved with SS, reassembled, put some fluid in & reinstalled. Without getting around to bleeding anything (system all new/empty), most of the MC fluid slowly leaked out… of BOTH ends of the MS.

Perhaps it was just loose fittings on the business end of the MC. But on the pushrod end- that's basically only 1 culprit; the seal on the piston. Mine was in excellent shape, pliable with no cracks or chips in the rubber, but perhaps it was compressed. I took the MC back out & apart.

In looking at diagrams in the manual, I have 2 'extra parts' Ford doesn't show: a thin steel washer between the piston & the primary cup, and a thick rubber washer at the very end where the fluid pushes out.



Should I take these out then?

Here's what is in mine :

 

The thin metal washer was a solution to keep the seal from sticking to the plunger so would leave it there. As for the thick rubber washer, it appears that it creates a seal for the valve that it sits next to. There are various manufacturer's of master cylinders and rebuild kits and each had their own version of the parts required. Neither of these parts would contribute to the leaks you describe.

 

Rickster- what is the function of the 'bleed holes' in the piston? The primary cup has indent in it's outer edge (visible above), they align if so indexed with the piston's bleed holes- is that necessary? (I can't see how).

I ordered a rebuilt kit online.

I think something rubber inside was shedding- there was a bunch of dirt inside the MC bore and it went together spotlessly. Puzzling….

 

The function of the bleed holes is to allow fluid that collects around the piston shaft to be released. The seal that is next to the thin washer is the main seal that creates pressure when you step on the brake pedal. It passes by the fluid port in the bottom of the reservoir and once past the port you will feel pressure build that applies the brakes. When the main seal passes the port, fluid can enter the area around the piston shaft because the reservoir port is now over that area. The other seal on the piston keeps fluid around the piston shaft from leaking out of the cylinder. The small indents in the seal are bypasses so the fluid caught around the piston shaft and released through the holes in the piston can escape past the seal and also works to keep the main seal clean. This design is all due to the pressures when you press on the pedal and when you step off the pedal and it needs to return quickly. The design allows the cylinder to work smoothly in both directions. As for the rubber that was shedding, I am not sure where that could come from unless you see obvious damage to either of the seals or to the rubber washer. You do want to check that the holes in the piston are clear.

 

10-4 on the above.

The 'thin washer' tho is the exact width of the bore, hence it seals over the piston bleed holes. That's why I was thinking it shouldn't be there, esp so with your above post.

Agreed that that's not any leakage cause, but might show up as problematic when working the brake pedal.

I saw nothing rubber deteriorated anywhere, and the MC was professionally cleaned & sleeved before I reassembled & reinstalled it. The piston I reused is sitting right next to me now, and it's rubber seal looks brand new (hence why I reused it). There also seemed to be slight corrosion specks on metal pieces inside; corrosion that would not wipe off but came off under a fine wire wheel. IE: rust. Brake fluid was opened a few months ago, has remained tightly capped other than when I poured some in the MC. Again: puzzled.

 

In some rebuild kits, the metal washer is actually impregnated into the rubber of the main seal. The pressure of the fluid going through the piston holes actually 'lifts' the washer off of the holes just enough to let the fluid by. The fluid can go through the center hole of the washer or around the outside. Understand that we are not talking about much fluid at all, in fact a very small amount so it can get around the metal washer and then by the rubber seal via the bypass indents in the seal.

 

The only way to be sure if the washer should be there or not is to refer to the rebuild kit assembly instructions. As I mentioned above, the kit manufacturers have various designs and theories of how to make the cylinder work properly.

 

I bought an NOS rebuild kit off of eBay. The new primary cup had an integral brass washer in it, so I deleted the large thin washer that was in mine & blocking off the bleed holes in the piston. I replaced the rubber washer all the way at the bottom and the valve in the bottom of the spring. Cleaned everything really well, re-assembled, tightened everything carefully (put all 3 lines into the Weatherhead block, then attached the WH block to the MC).

Put some fluid in and zero drips or wetness overnight. Will pump the piston a few times and watch it further… but feeling more confident that it's set right this time.

 

Sounds like you are on track to getting it working with no leaks. The impregnated brass washer takes the place of the steel washer as you have discovered. Good luck with getting your brakes fully operational.

 

Most ebay sellers haven't a single clue what parts fit, even if the box has a part number on it.

So, what part number did you buy?

 

Was a 'universal kit', 1 1/4-in bore, fit numerous trucks.

Here's the link :
http://www.ebay.com/itm/291740755879?_trksid=p2060353.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT

 

Where did you have the MC re-sleeved?

 

Brake & Equipment Warehouse, Minneapolis MN.
Nice work, speedy turnaround.

 

Brake & Equipment Warehouse, Minneapolis MN.
Nice work, speedy turnaround.