Clutch MC seal gets stuck

[The Veg]

I just rebuilt my clutch master cylinder. The old seal wasn't terrible, but a small amount of fluid was getting around it. The new seal, however, has a worse problem- it gets thoroughly STUCK in the cylinder bore. I've done the following:
-Made sure that it's properly installed
-Made sure that it's got plenty of fluid coating it
-Made sure that the bore is clean, smooth, and free of pitting
-Soaked MC in diesel 24 hours and wiped it again just to make doubly-sure
-Checked seal dimensions with supplier, who conforms they are correct and that I got the right kit for a Girling 5/8" cylinder
-Ensured that the edge of the seal is not folding upon insertion

The seal still persists in getting stuck, either not very far in or, if it goes further, at any other point in. I have to beat the cylinder hard on the bench to get the piston back out.

What could I be missing here or doing wrong?

[patrics]

Hi,

"Soaked MC in diesel 24 hours and wiped it again just to make doubly-sure"
Have you really done that?

Regards
Steve

[Mick6186]

When the cylinder is installed in circuit with the clutch the return pressure of the clutch will easily overcome friction in the master cylinder bore & return the piston,
Mick

[Craven]

Some sort of distortion of the bore, slightly oval!

[The Veg]

"Soaked MC in diesel 24 hours and wiped it again just to make doubly-sure"
Have you really done that?

Yes. Removed it from the car and soaked it with the piston removed, of course.

[2cams70]

What kind of brake fluid are you using? Don't use that silicone stuff. It's poor lubricity can cause the problems you describe.

[nmauduit]

What kind of brake fluid are you using? Don't use that silicone stuff. It's poor lubricity can cause the problems you describe.

well, on several street vintage vehicles I use DOT 5 silicone hydraulic fluid, and though not as viscous as DOT 5.1 it has some lubricating properties.
It also is non corrosive, does not absorb water, and boils at 260?C. Only drawback I see is cost.

as for the slave cylinder in question, since all dimensions have been thoroughly checked, has some brake grease been used for reassembly (or at least the hydraulic fluid to be used)? if not the actual force required to move it should be reassessed after application of such fluid.

[2cams70]

What kind of brake fluid are you using? Don't use that silicone stuff. It's poor lubricity can cause the problems you describe.

well, on several street vintage vehicles I use DOT 5 silicone hydraulic fluid, and though not as viscous as DOT 5.1 it has some lubricating properties.
It also is non corrosive, does not absorb water, and boils at 260?C. Only drawback I see is cost.

Silicone fluid is definitely not the be all and end all of brake fluids. Definitely has it's disadvantages that aren't just cost based. OEM's generally still use glycol based fluids and not silicone.

http://www.adlersantiqueautos.com/articles/brake1.html

[rgh0]

A new seal will normally not move freely in the bore. Worn seals move freely ---- because they are worn. The master cylinder will return under pressure from the slave cylinder when released. if it does not it is normally due to the end seal piston back to the reservoir leaking.

i would check the bore diameter and seal to ensure what I have is correct. I would replace the seal as I dont know what diesel fuel does to a hydraulic cylinder seal rubber but it may not be good and may cause softening or swelling. I would lubricate with a good rubber grease on assembly and all should be fine

cheers
Rohan

[Elan45]

I think the biggest culprit is your soaking of the master cylinder in diesel fuel. Why????
If you want to clean a brake component, it should be done in denatured alcohol or the commercial "brake clean" products. I usually clean in alcohol for a first stage, then follow up with a liberal spray of brake clean product. My father taught me at an early age to never expose brake components to petroleum products.

I would suggest you buy a gallon of alcohol and soak your MC in about a quart of this solvent. After a week, dump it out, clean everything and repeat the alcohol soak again. Finally, spray it w/ brake clean and let it air dry. Buy new rubber components and install w/ Girling rubber grease on the rubber. Since you've probably contaminated the hydraulic lines and the slave cylinder, these components need cleaning also.

I've cleaned hydraulic lines by pouring alcohol into the line to fill it, then hanging it in the garage and finally blow out w/ compressed air. You might also use brake clean here too.

Roger

[patrics]

Hi

"Soaked MC in diesel 24 hours and wiped it again just to make doubly-sure"

I am not sure how you get over such massive contamination like this. The obvious and easiest thing to say is to throw the master cylinder, slave cylinder and pipework in the bin and start new.

You could try cleaning it as Roger suggests but you might not be successful ? it is not straight forward as the cleaner / degreaser for a mineral substance would also be mineral and so the contamination continues.

Your title ?clutch MC gets stuck? does sound like contamination.

Regards
Steve

[The Veg]

The sticking happened before the diesel. When soaked in diesel (which I'm told is the closest thing to the 'paraffin' that you guys suggest soaking parts in to cure damn near everything that doesn't move properly), the cylinder was stripped of everything but its metal casting so the seal was never exposed, not that I would expect any harm anyway given the stuff's relative docility toward most polymers. But I still gave it a good cleaning afterward.

I don't doubt that the clutch pushes back- but I've never rebuilt any hydraulic cylinder in my life -brakes and clutch cylinders both- that didn't move relatively freely with a new seal, at least freely enough that they didn't require giving the cylinder a STRONG whack against the bench to get the piston to return to its neutral position, so something seems amiss.

EDIT: I failed to point out too that it isn't just getting stuck, it's very difficult to push in at all and was so even when the new seal had seen nothing but a coating of DOT 3.

Maybe I should just eliminate all doubt and start over with a new cylinder.

[nmauduit]

Maybe I should just eliminate all doubt and start over with a new cylinder.

well, depending on your urge of using the car and time you are opened to devote to a new experiment, you may as well give it a try and see if it works... it's not that difficult to remove again in case it indeed does not operates as it should (there is some resistance in a normally operating hydraulic component, just consider the pressure the seal has to take in the brake system for reference).

----

ps: I initially somehow got the impression it was the slave cylinder that was concerned, but regarding the MC the internal spring should be strong enough to make it go back to the resting position against the stop without the need of a wack if lubricated with brake fluid. If it is not the case, I would double (triple ...) check the assembly and parts (small tear of the rubber, something sticking out of a hole into the bore etc). Hydraulic can be challenging at times (esp. the intermittent problems ...)

[MarkDa]

The outlet to MC is clear isn't it?
If the piston won't go in or out it may be pressure or vacuum causing the problem?

[The Veg]

No pressure issue, as it will squirt readily from the port with everything disconnected.

Actually I think it was a cleaning issue. I had said that it looked clean and smooth, which it did, and felt smooth in the portion that I could get a finger into. Nevertheless I took the advice to try alcohol. Gave it a long soak in that and scrubbed the bore with an old toothbrush. Nothing appeared to come out of it at any point, but it works much better now. Mounted it in the car, and it still sticks slightly but nothing that a sharp pump on the pedal won't overcome. I suspect that the return-pressure of the clutch will over come that as well, but before I can test that theory I've got to stop all the banjo-fittings from leaking. Apparently, at least on the MC, the mating surface that the copper washer bears against needs to be dressed and despite the SC being new, the washers won't seal on it either. But that's another discussion.