Clutch operational issues (theory questions)

So I have installed a new Tilton brand clutch hydraulic mastercylinder in my Nomad restomod. Custom fabricated mount position on the firewall, pushrod and pedal attachment under the dash.

Its plumbed using a stainless line down into the T56 transmission, with a stock hydraulic throwout bearing.

The clutch works well for the usual start/stop and upshift/downshift requirements of driving.

But when I push the pedal in, and hold it for extended periods (like 1+ minute), the throwout bearing seems to be slowly releasing...such that the clutch is no longer engaged. Almost feels like maybe the fluid is not "holding" the TO bearing in position after holding the pedal down for 1+ minute continuously.

I see no evidence of any fluid leaking out of all the connections in the system, and the small fill tank is full of fluid.

What could be allowing the fluid pressure to back off slowly?

When it does this and I release the pedal...the pedal will not return to it's normal position. It slowly backfills the pressure and resumes correct operation within about 5 to 10 seconds of releasing it.

Ideas on what's going in here? I thought I bled this thing of all the air when I initially filled it for the first time (a year ago), but maybe there's still air trapped in the system?

Your problem of losing the throwout bearing stroke would seem to be leakage of fluid past the master cylinder seals back into the reservoir. The symptoms are opposite what they'd be if you had air. If you had air, the clutch wouldn't ever release. With yours, it does then loses it. If the throwout bearing was leaking, it would be an external leak. The master cylinder can leak back to the reservoir with no external leak. It takes very little volume loss to lose significant pressure.

The problem of the clutch pedal not coming up is probably due to having an orifice somewhere between the master cylinder and the throwout bearing. Since you have an aftermarket master cylinder, it's not there, it's somewhere in the GM parts you still have. GM put it there to keep you hotrodders from doing those jackrabbit starts and hard shifts, to lessen their warranty risk. A cheap means of torque control. You can drill out the orifice. Do some googling, you'll probably find out where it is, what size the hole is, and what size you can drill it to.

I did some googling myself here, as I have a similar setup with a Tilton master cylinder (as opposed to the GM one) but a stock type GM throwout bearing.

What I found out was that an OEM clutch master setup has the orifice in the hose assembly that connects the master cylinder to the hydraulic throwout bearing. The fitting at the master cylinder end has a 1/16" orifice, and the "fix" is to drill that fitting out using a 1/8" drill bit.

What's puzzling here is that my Tilton m/c has a standard -4AN adapter screwed into the outlet port, and a standard -4 hydraulic line. No orifice in the usual spot, and no GM parts there. Is yours that way?

Anyway, I'd still be looking for a restriction.

FWIW, I bought my master cylinder setup from Tick Performance.

Sounds like you have an internal leak either in your MC or your throwout bearing. To isolate it I would remove the line from the MC, cap it, and stand on the clutch pedal to see if it drops over time, like for a few minutes. If the pedal drops, the MC is leaking internally. If it doesn't, the problem is in the throwout bearing. The fact that the clutch pedal doesn't return correctly also indicates to me that fluid is getting past the piston in one or the other location, and has to flow back to where it's supposed to be.

I don't think you can have an internal leak in the throwout bearing. One side has brake fluid, the other side is vented to atmosphere. So it's going to be an external leak if there's a leak.

I think I'll try Chevynut's method to isolate the clutch master cylinder for evidence of "fluid blow by". This is the 2nd Tilton MC I've put into it, the first one never even made it to the point of the car running on the street as my pedal/rod adjustment was too long and it damaged the seal inside. So this one is also a new one, and the pedal NEVER bottoms out...the way I have the linkage installed. But I too suspect the fluid is getting by the plunger inside the MC...

Update:
Once again, operator error here. :eek:

When I fabricated and welded in the new firewall, I only tacked it around the edges.
Fast forward a year or so now (of actually driving the nomad short distances around town) and what happened is the spot welds at the firewall overlap began to break near the mounting area of the clutch mastercylinder. Such that the whole MC was pushed 1/4 inch forward of its original mounting position (because of firewall sheet metal flex).

the clutch problem was progressively getting worse with time, and I finally inspected up under the dash to see the firewall flexing when the pedal was pushed in. Paul...you dumbass!

So obviously, 1/4 inch makes a difference in a hydraulic clutch system. Such that it was barely engaging the TO bearing when pushed to the floor.

I have since fixed the welding and strengthened up that area of the firewall, bled the system and it seems to be working fine now.

But I'm still going to plug the line coming out of the MC and do the leakdown pressure-pedal pump test to verify that fluid is not transfering back into the reservior when the pedal is pushed and held down. Still waiting for a 3AN plug fitting to show up at my door, and then it will be a quick test.

Rookie build techniques here. A little embarassing, but wanted to post the outcome anyway. :(

If you are going to remove the line and plug the master cylinder outlet (with the subsequent need to re-bleed), I'd double check that the line doesn't have a restriction/orifice in it as part of this exercise.

What did you use for a bracket to mount the master cylinder?

Good catch on the firewall movement. 1/4" will make a serious difference whether it's hydraulic or mechanical. The stock firewall/pedal brace isn't too bad on a 55-57, but I've had to reinforce an early Camaro in that area in the past, with just a "normal" clutch.

Quote:

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What did you use for a bracket to mount the master cylinder?

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I didn't reinforce with anything. Just drilled thru the 16 gauge sheetmetal firewall. But it's within 1 1/2 inches of the stock inner hood hinge flange (inside of firewall) that's welded to the firewall, so it didnt push itself straight forward, but bowed outward towards the center of the car (weakest point of the firewall when pushing)

So now I have welded new firewall to the lip of the old firewall much better, and added a 1/8 plate on the inside of the firewall...to spread the load across the sheetmetal better. Seems more solid that way.