Clutch master cylinder question
10-23-2017, 07:58 AM
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Clutch master cylinder question
1992 3VZ-E, soft clutch action, so I replaced the slave and master cylinders. I can't seem to get the new master to drive fluid through the system. I've primed the lines and slave using a hand-held vacuum pump, but when I cycle the clutch pedal to bleed the system, nothing comes out the bleeder valve. Reservoir level remains unchanged with repeated pumping. I want to conclude that the brand new master cylinder is bad, but before I take it back and get all huffy, is there any need to somehow prime the master cylinder first? It doesn't seem like anyone else is having this problem.
Also, when I bench test the master cylinder, and actuate it by hand, I get a single spurt of fluid, then almost nothing with repeated cycles. But wait 30 seconds, and then I get a single spurt again, then almost nothing. If I plug the output with my finger, it generates and holds good pressure. Just doesn't seem to be moving any fluid. I pulled the reservoir and cleaned the inputs. Anyone know what volume this thing should be putting out on a bench test?
Also, when I bench test the master cylinder, and actuate it by hand, I get a single spurt of fluid, then almost nothing with repeated cycles. But wait 30 seconds, and then I get a single spurt again, then almost nothing. If I plug the output with my finger, it generates and holds good pressure. Just doesn't seem to be moving any fluid. I pulled the reservoir and cleaned the inputs. Anyone know what volume this thing should be putting out on a bench test?
10-23-2017, 12:28 PM
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Maybe. I don't see a procedure for that in the FM and can't find any references online. But I did fill the reservoir, on the bench, and cycle the plunger a few times. No sure how else to bleed it. Again, when I do that, I get a decent squirt from the output port, then nothing but small spits. If I wait about 30 seconds, same thing: one good squirt and then spits.
10-25-2017, 01:41 PM
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You are plugging the slave cylinder side of the line after pressure is applied to the clutch pedal and keeping pressure there until the pedal is released yes?
10-27-2017, 02:50 PM
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OK, got it all fixed up. I'll write it up here for posterity, in case anyone else has similar problems.
Problem was a soft clutch and some minor but messy leaking fluid. I replaced both the master and slave cylinders, and the rubber hose that connects the two metal lines. All those parts are cheap, so might as well do it all. When assembled, problem one was that actuating the clutch pedal did not drive any fluid through the system. I bench tested the master by shoving the pushrod with the reservoir filled. That gives you one good squirt followed by some little spits. I thought it was a bad unit, but now realize that's expected behavior as the master isn't really a pump, it's more of a single shot actuator. The inlet hole from the reservoir is small, and it takes 20 to 30 seconds to fill the chamber. So one take away is when your are bleeding the system, using the clutch pedal, you should wait 20 seconds between each pedal cycle.
I primed the lines by applying vacuum at the slave cylinder bleed valve and attaching a hose from the master-cylinder-end of the line, with the free end of the hose submerged in a can of fluid. No problems filling lines this way. Then I attached the primed master, and bled the system by pumping the brake pedal and draining the lines through the bleed valve at the slave. My "lovely assistant" isn't sympathetic to my multi-day repair jobs, so I open the valve a tiny bit, then cycle about two reservoirs worth of fluid through the system, then close. (Brake fluid is cheap, and well worth the cost of not bothering my wife...).
But.. it still didn't work. So, I bled the slave cylinder as well, by closing the bleed valve at the slave and forcing the actuator closed. This is easy on the 3VZ, you just take a 3 inch C-clamp and put one end over the throw-out bearing lever arm and the other end around one of the mounting hole bosses. Takes about 30s. Now, any air in the slave is pushed back up into the lines. Next, open the bleed valve, with the slave compressed, and bleed the system using the clutch pedal as before. Close the bleed valve and release the C-clamp and now... it works!
I think I installed the slave without priming it, and some air was trapped therein. The bleed valve is located on the top side of the slave cylinder, so you would think the air would bleed out first, but my experience was otherwise. As easy as it is to compress the slave, I'd recommend doing that as a first step to bleeding the clutch system anytime it's done.
Also, there are a lot of videos that show you how to bleed the system from the bottom, by compressing the slave and watching for bubbles in the master cylinder reservoir. I measured the reservoir volume at 50ml. When I used compressed air to blow out the lines through the slave bleed valve, I recorded about 30ml. I filled my old slave and measured it's capacity at 18ml. My 30ml number could be high, since it might include some fluid from the cylinder. But all this implies there's not enough volume in the slave to push all the fluid in the lines back up into the reservoir. That means the bottom bleeding technique probably isn't 100% effective. If somebody wants to pull some stuff and get a more accurate measure of the line volume, I'd love to hear about it.
Problem was a soft clutch and some minor but messy leaking fluid. I replaced both the master and slave cylinders, and the rubber hose that connects the two metal lines. All those parts are cheap, so might as well do it all. When assembled, problem one was that actuating the clutch pedal did not drive any fluid through the system. I bench tested the master by shoving the pushrod with the reservoir filled. That gives you one good squirt followed by some little spits. I thought it was a bad unit, but now realize that's expected behavior as the master isn't really a pump, it's more of a single shot actuator. The inlet hole from the reservoir is small, and it takes 20 to 30 seconds to fill the chamber. So one take away is when your are bleeding the system, using the clutch pedal, you should wait 20 seconds between each pedal cycle.
I primed the lines by applying vacuum at the slave cylinder bleed valve and attaching a hose from the master-cylinder-end of the line, with the free end of the hose submerged in a can of fluid. No problems filling lines this way. Then I attached the primed master, and bled the system by pumping the brake pedal and draining the lines through the bleed valve at the slave. My "lovely assistant" isn't sympathetic to my multi-day repair jobs, so I open the valve a tiny bit, then cycle about two reservoirs worth of fluid through the system, then close. (Brake fluid is cheap, and well worth the cost of not bothering my wife...).
But.. it still didn't work. So, I bled the slave cylinder as well, by closing the bleed valve at the slave and forcing the actuator closed. This is easy on the 3VZ, you just take a 3 inch C-clamp and put one end over the throw-out bearing lever arm and the other end around one of the mounting hole bosses. Takes about 30s. Now, any air in the slave is pushed back up into the lines. Next, open the bleed valve, with the slave compressed, and bleed the system using the clutch pedal as before. Close the bleed valve and release the C-clamp and now... it works!
I think I installed the slave without priming it, and some air was trapped therein. The bleed valve is located on the top side of the slave cylinder, so you would think the air would bleed out first, but my experience was otherwise. As easy as it is to compress the slave, I'd recommend doing that as a first step to bleeding the clutch system anytime it's done.
Also, there are a lot of videos that show you how to bleed the system from the bottom, by compressing the slave and watching for bubbles in the master cylinder reservoir. I measured the reservoir volume at 50ml. When I used compressed air to blow out the lines through the slave bleed valve, I recorded about 30ml. I filled my old slave and measured it's capacity at 18ml. My 30ml number could be high, since it might include some fluid from the cylinder. But all this implies there's not enough volume in the slave to push all the fluid in the lines back up into the reservoir. That means the bottom bleeding technique probably isn't 100% effective. If somebody wants to pull some stuff and get a more accurate measure of the line volume, I'd love to hear about it.
