Clutch master cylinder failure

Hi All

Thought this was interesting. I bought a new clutch master cylinder for my 1960 Land Rover. I have swapped these in and out and rebuilt or replaced them many times over the years. This is the first time I’ve seen one fail this way, and it is brand new out of the box. In my opinion, the seal at the back of the cylinder that is supposed to close off the reservoir from the inside of the cylinder, well, isn’t. So, it is pushing fluid up into the reservoir instead of down through the lines. Please have a look at a this video I made and let me know what you think. Since I already know what I think :slight_smile:

I believe that the problem is at the throw out bearing and pressure plate fingers. When there is no return pressure at the pedal look at the clutch fork. It will likely be fully extending the throw out bearing and binding because it has over extended.

I agree. The cup seal within the master cylinder is allowing the hydraulic fluid to leak past it, and instead of applying hydraulic pressure down the line the fluid is being displaced into the reservior. Was this a remanufactured master cylinder?


I think there is a BUUUNCH of air trapped in that funky ‘crazy straw’ clutch line. Look at the video at the 1:49 mark. As the line runs to the slave cylinder, it curls around, with the curl that looks like it is above the master cylinder and the possible location of the slave cylinder. Basically, it is the highest point in the entire system. That rigging seems to me to be a perfect air trap in the line. And I have no idea how to possibly get the air out of a line bent around like that. But, air compresses, and enough air trapped in the line would make the pedal feel fairly dead. It would also explain the funky rising and falling fluid in the reservoir.

I coiled the line to get rid of some excess in the line as this M/C was a retrofit done as part of a single to dual circuit brake conversion. The setup has worked perfectly for years, decided to swap in a brand new M/C while the fender was off and things were accessible. @ Bustedknuckles, the air will get pushed out of the coil of pipe just fine as long as the bleed screw is open. Sure you can compress air, BUT energy follows the path of least resistance and that is a n indisputable law of nature. it is far easier for the air bubble to push the fluid along the line and out of the slave cylinder as long as the bleed screw is open, i.e., the path of least resistance. On the odd occasion that you fuss with the pedal and it works properly, everything functions as normal and you can watch the slave cyl. pushrod move the fork. Why would air in the line downstream from the reservoir force brake fluid past a perfectly good seal to rise up in the reservoir. If the seal was working it would be able to resist 100’s of PSI of hydraulic pressure. A few psi of compressed air should not be a problem. And if it was, it would not be intermittent the way this is. But, I do appreciate the input and don’t mind the comment on my brake line handiwork. I built this car in 1999, probably would do it differently now :slight_smile:

@ Rod Knox, when the pedal doesn’t return, it is because the slave cylinder has barely moved and there is no pressure from the clutch fingers pushing back. I just put the engine in and the clutch is brand new and the throwout assy. totally rebuilt.

@ tester, brand new TRW cylinder.

thanks guys! BTW I totally love the user names on here. I feel like I fit right in :slight_smile: