Guess I need a new seal

[Welder Dave]

The new slave cylinder is working but is leaking. Napa has the piston seal for $4.19 so that's not a big deal. I've been reading lots of new slave cylinders on different vehicles leak from new. I think I'm going to get some special grease compatible with brake fluid to smear on the bore and the new seal to have the best chance of success. When I had the slave apart there were a couple vertical manufacturing lines in the piston seal almost like a crease. Not real visible but they were there. I'm wondering if that could be enough to cause it to leak? The other thing I'm thinking about doing is putting a washer or some kind of spacer on the fluid side of the seal so it can't retract quite as far where the lip of the seal is partly over the ports for the hose and bleeder screw. Moving it down 1/8" might be all that's needed. There's a spring but it's pretty weak.

 

[56wrench]

You should run a hone through it before you install a new seal and see if any score marks are there

 

[Welder Dave]

I'm wondering if it could be a bad seal or it got knicked putting it back in dry. Also if I damaged it when bleeding. I had to pump quite a bit and then forced the pedal down really tight with a board. I think having a helper is much better.

 

[Welder Dave]

I think the best thing to do is get a new seal (it may have been bad from the start), hone the slave (maybe welding distorted the bore or it was too rough to begin with) and use brake grease on the piston seal and bore of the slave (maybe I damaged the seal). I was reading where someone said the reason new wheel cylinders leak is because they weren't greased before being put into service and another that said it's important to grease slave cylinders. They use the same type of cup seal. Might not all be needed but since I have to take it apart don't want to scimp on the little details and have to take it apart again. Hoping I see some damage on the original seal to explain the leak.

 

[56wrench]

If this system uses DOT3 brake fluid, it usually works for reassembly lube but there is a special slippery lube available but make sure it is compatible with the the fluid used in the system. Master cylinders, wheel cylinders and slave cylinders should never be assembled dry

 

[Welder Dave]

I may have buggered the seal. It seemed pretty stiff compared to the old seal I took out that blew. That seal is more pliable and has a more consistent curve shape to sealing edge. It split in the bottom where the internal spring pushes on it. The piston was almost stuck in the bore half way out because of so much rust and crap in it. I had to twist it side to side and yank on it pretty good to get it out. Permatex has brake compatible grease and I've seen small pocket packs of Syl-Glide that is brake grease. Hoping I can find one of the little packs.

 

[Welder Dave]

I got a new made in USA seal, actually the same as the seal in the old slave, a little packet of Permatex orange silicon ceramic brake grease and a brake wheel cylinder hone. Supposed to be decent weather tomorrow so going to try and get it all done. The one thing I'm trying to figure out is how to keep the piston seal from retracting too far where it's partly over the ports. Maybe with everything lubed the internal spring will keep it pushed down or at least push it down if I pull the piston out slightly. Hoping once it's got some fluid in the system it will keep it down. I can see the seal sticking when it was dry.

 

[Welder Dave]

I think I have it fixed, at least it didn't leak the little bit I moved it back and forth. I honed it and applied brake grease to the bore and piston seal. I also didn't pump it up where the pedal was rock solid. I slowly pumped it about 5 times and then put a board on the pedal and opened the bleeder. I did this 3 times and tried it out and it works. There's a few things I think may have contributed to the leak. The bore may have been slightly out of round from welding the new ports on, there was a slight imperfection in thd bore that knicked the seal, the seal had a minor flaw or was too stiff to seal properly and/or I over pressurized the slave by pumping the clutch too much when bleeding it. Here's some pics. of the leaky seal and new seal. The new seal is a little lower in height and the sealing lip is a little thinner that maybe makes it seal better. The camera makes the tiny knick on the seal much more noticeable than looking at with your bare eyes. You wouldn't think a tiny knick would cause a leak but maybe or it's a combination of things.

 

[56wrench]

Also, the shorter seal may not partially cover the port?

 

[Welder Dave]

Yes, I remember when I first got it, I could see the lip of the seal part way over the ports. That 1/16" or whatever may make a difference. The other thought is perhaps the hole for a port is what caused the knick either rubbing on it or when pressure is applied it pushed the seal inward for a split second. I did look at it after honing and it doesn't look like the seal gets too close to the port with the internal spring in place. When I took the seal out for welding I did notice a line on the seal. It may have been the issue from the start even if I didn't have to modify the slave. I mistakenly thought the slave would retract all the way to the snap ring that holds the return spring on the piston. It doesn't although you can manually push it in all the way to the snap ring. When I put the slave back together after welding I thought I'd have to force the bottom guide in the bore because of the spring so I could install the snap ring. It was a little weird because it just "popped" in and stayed there. Possibly a bore issue. Putting it back together after honing and lubing I did have to hold it in with a punch so I could get the snap ring on. It could have been a few tiny issues that caused the leak. I'll still monitor it but I think it's good and I didn't go crazy pressurized it to bleed it.