Is this a problem with the clutch hydraulics?

[martin calva]

Hello,

Several times, with weeks between each episode, I have had a problem with the transmission of my manual 940 diesel.

When driving at low speed, the clutch pedal has gone straight to the floor with no resistance. and I cannot get the car out of its present gear into neutral. Pumping the clutch pedal has no effect. So I have braked to a halt and switched the engine off and put the car into neutral. On restarting the engine, things seem to be back to normal and the car has then driven normally until the next episode of the problem, usually weeks and 100's of kilometres later.

My guess is that there is a problem with the hydraulic system of the clutch, which I am currently overhauling (new master cylinder currently being fitted, overhaul kit for slave cylinder standing by).

Is there any explanation for the problem other than the hydraulics? If not, then I can be confident that I'll have cured the fault. Otherwise maybe the problem needs further investigation...

My thanks for any ideas or advice.

Martin

[cheshired5]

They can be a nightmare to bleed without a pressure bleeder so I would recommend using one to refresh the fluid and eliminate any air.

I've restored a few clutch pedal operations with a pressure bleed.

[martin calva]

Originally Posted by cheshired5

They can be a nightmare to bleed without a pressure bleeder so I would recommend using one to refresh the fluid and eliminate any air.

I've restored a few clutch pedal operations with a pressure bleed.

Thank you for the advice. In fact I have already started to remove the master cylinder but purging the system of air with a pressure bleed would have been quick to do. However, as the problem only happens now and then, it would not have been obvious whether or not the problem had been dealt with.

[Familyman 90]

Sticky master cylinder.

[AllHailKingVolvo]

I had this issue with the first 940 I owned, quite a while ago now. Pedal would go to the floor and stay there until I could hook my foot under it and lift it up, at which point it would return to normal function. Very odd!

I cured it by changing the brake/clutch fluid (they share a reservoir as you probably know) and flushing then bleeding the system. Whether or not this was the cure per se or not I don’t know, but it stopped happening after I changed the fluid and the problem didn’t recur. The old fluid was very dark grey and dirty, so it was well overdue a change.

I rebuilt the clutch MC on my diesel 940 earlier this year. Getting it out is a PITA (as you found-I read your other thread!) but the actual rebuild process is fairly simple and self explanatory. Just remember to make a note/drawing/photo of the orientation of the old plunger seals before you take them off…they’re very easy to install the wrong way around!

[Bugjam1999]

Possibly the rubber hose breaking down inside, they can act strangely when this happens, like 1 way valves. Cheap to replace and it sounds like you’ll have the system in parts soon anyway.

I’ve got a Sealey pressure bleeder for brake and clutch work, it’s a brilliant bit of kit, really easy to use and well worth the £45 or so I paid for mine - this ad is slightly more, you may find a lower price elsewhere.

https://www.ebay.co.uk/itm/Sealey-Ca...-127635-2958-0

Cheers

[martin calva]

My thanks to all for the helpful replies.

I replaced the master cylinder and I replaced the rubber parts of the slave cylinder and then I pressure bled the system. I did not replace the tubing (it's a single unit on a LHD car, comprising a rubber hose between tubular metal ends and costing around 85€ at Skandix).

The problem has not reappeared so I hope it has been resolved. What caused the problem remains a mystery - both master cylinder and slave cylinder and their associated rubberware seemed to be in good condition with no sign of binding anywhere.

[Laird Scooby]

Originally Posted by martin calva

My thanks to all for the helpful replies.

I replaced the master cylinder and I replaced the rubber parts of the slave cylinder and then I pressure bled the system. I did not replace the tubing (it's a single unit on a LHD car, comprising a rubber hose between tubular metal ends and costing around 85€ at Skandix).

The problem has not reappeared so I hope it has been resolved. What caused the problem remains a mystery - both master cylinder and slave cylinder and their associated rubberware seemed to be in good condition with no sign of binding anywhere.

The master cylinder problem may have contributed to your 3rd gear sunchro problem in the other thread................

[martin calva]

Originally Posted by Laird Scooby

The master cylinder problem may have contributed to your 3rd gear sunchro problem in the other thread................

Dave, I wondered that.

However:

- The hydraulic problem meant that sometimes I could not disengage the clutch and the pedal seemed to offer no resistance as if all hydraulic fluid had been lost, so I'd bring the car to a halt and switch off. At all other times, never the slightest problem engaging or diengaging any gear. I never made any attempt to use force to disengage a gear so hard to see how the clutch issue could cause harm inside the gearbox.

- The third gear problem appeared for the first time yesterday and now, every time I engage third, it quickly jumps out of gear if I am applying power. It's at least a couple of weeks since the last appearance of the clutch problem.

[classicswede]

The problem you describe is most likely the return spring inside the master cylinder. Not common but I have seen a few and it does exactly what you describe