• Hello Guest, welcome to the initial stages of our new platform!
    You can find some additional information about where we are in the process of migrating the board and setting up our new software here

    Thank you for being a part of our community!

740 hydraulic clutch dead, how to fix?

7

740_lower

New member
Joined
May 1, 2024
Location
Nordics
Hello, the clutch bite point on my 1991 Volvo 740 has progressively gotten worse over the last months. Car gradually got more "jumpy" when getting into 1st gear from a standstill

Last week the clutch disappeared completely. When starting the car, it jumps forwards on the starter motor, even though I press the clutch pedal in. So the clutch seems to be engaged all the time, making the car unsafe to drive..

Laying under the car, the slave cylinder moves while the clutch pedal is depressed, but it seems weak? Tried to put a nut between the slave cylinder and fork to increase the throw (in case the pivot nut was worn), but then the cylinder almost wouldn't extend anymore when depressing the pedal..

I understand that the clutch fluid reservoar is the same box as the brake fluid reservoar (pizza slice shaped box), albeit with separate chambers inside the white box

Do you have any tips on how I can bleed the clutch hydraulics, or what else might be wrong?
 
You probably need to replace both the master and slave cylinders. I write that because when one fails the other isn't far behind. When bleeding the system it's just like bleeding the brakes. Just be sure the reservoir doesn't go to low where the hose comes out to goto the clutch master cylinder to avoid sucking air into the system. Check the condition of the rubber flex hose. If that is compromised time to replace that as well.
 
Look under the dash where the clutch master comes through the firewall. Any wetness there? A worn-out master will often leak fluid from the pushrod area, rather than sending it to the slave. And of course, a worn out slave can leak in the same way. That's assuming there's enough fluid in the reservoir, and it isn't sucking air.
 
Will have a look later today for leaks under the dash. But reservoir levels are on the max line, and haven't been touched for years.

Eager to order new parts, but not sure which part has actually failed. Clutch hose is currently out of stock, so waiting a little

The brake pedal has also been sinking to the floor a little the last year or two, hope that will improve with a new master cylinder
 
There is still the chance that it is the clutch itself. Or maybe the fork has worn a hole thru it, or the fork pivot pin has worn down.

I forget exactly how much the fork travels, but if it is moving about 1/4" it may be doing its job.

It is possible some air got in the system somehow. I use a vacuum bleeder. It also can help to compress the slave cylinder while bleeding.

I have also had slave cylinders travel too far due to clutch wear and puke some fluid and get air in the system.

I have a M90 in my 740 and I had to guess on the shim size for the pivot pin. I had the first slave cylinder travel beyond its normal range and I think
the seal tore and it puked fluid and I lost the clutch. A new slave cylinder fixed the issue
 
740_lower said:
Will have a look later today for leaks under the dash. But reservoir levels are on the max line, and haven't been touched for years.

Eager to order new parts, but not sure which part has actually failed. Clutch hose is currently out of stock, so waiting a little

The brake pedal has also been sinking to the floor a little the last year or two, hope that will improve with a new master cylinder
Click to expand...
A radiator shop should be able to make you a new hose. got one made in town for 50 bucks, picked up next day
 
ZVOLV said:
There is still the chance that it is the clutch itself. Or maybe the fork has worn a hole thru it, or the fork pivot pin has worn down.

I forget exactly how much the fork travels, but if it is moving about 1/4" it may be doing its job.
Click to expand...
Thanks, the slave cylinder moves the fork a little (while operating the pedal), but seems to not push any further once the fork gets heavy.

Tried to shim with a nut, in case the pivot ball was worn. But nevertheless, the rear wheel still wouldn' turn freely while in 1st gear and clutch pedal to the floor. And I noticed the slave wouldn't extend as far as withoutvthe nut, i.e. the force on the fork with or without the nut inbetween 'seems' comparable.

Based on this, Weak hydraulics is my suspicision.


Also tried to operate the fork by hand (while in 1st gear), but real wheel still won't turn freely (left side of car is lifted)

Was considering using a wrench to get more leverage on the fork (to confirm the clutch is working), but I'm a little afraid to bend the fork.
 
How heavy is the fork supposed to be?

Based on the clutch pedal leverage, it does seem like it would be too heavy to function test with bare hands
 
Last edited:
Pietercc said:
A radiator shop should be able to make you a new hose. got one made in town for 50 bucks, picked up next day
Click to expand...
Great tip. Will see if there is a shop nearby that can make one

I'm considering buying a new master and slave cylinder, but try with the old hose first

Just going to lay down first and see if I can detect any obvious bulge.. :)
 
Hose looks good. No bulging during operation of the pedal, and pretty consistent look throughout (only a fine layer of dust, and a little corrosion on the nuts)
 
Just realized that the clutch master cylinder is a separate unit from the brake master cylinder o_O Good I haven't ordered parts yet :-)
 
Have loosened the clutch main cylinder from inside the cockpit

It's a little tight on the engine compartment side (lots of wires etc). Any tips on how to get it out without making a mess?

Think I'll put some towels, and will try to take the reservoar hose off the clutch cylinder, and tape it above the reservoar..
 
You must log in or register to reply here.
Back
Top