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MG MGB Technical - Bleeding the clutch

1970 mgb - The Clutch was working fine, but I had a leaking master cylinder seal.
I have just replaced the seal, replaced the unit and cannot bleed the system.
The oil is passing thru and a the clutch pressure has increased slightly. If I pump the clutch pedal a squirt of hydraulic oil comes out of the master cylinder reservoir (Straight up out of the top where the cap should be).
What have I done wrong.
At least it will be easier to get it apart the second time.
D G Levy

Put the seal on back to front??
Chris at Octarine Services

I can't see how you get a jet of fluid (not oil) out of the filler if the fluid level is correct. You *will* get fluid behind the pressure seal as it passes the bypass hole as the pedal is operated, that's why there is a secondary seal closer to the end of the push-rod. But in theory that should be just a static quantity moving back and fore with the piston i.e. not moving in and out of the bypass hole while the piston is moving. As the pedal returns fully the pressure seal covers the bypass hole then clears it again, this time in front of the seal, so I suppose if there were any residual pressure left in the line to the slave, possibly from air, it could cause a spurt of fluid, but as I say I wouldn't expect that to be visible with the master correctly filled. Only if the secondary seal was somehow jammed in the bore, but the pressure seal was still moving with the piston, would you get fluid pumped in and out of the bypass hole as the pedal was operated and released.

If you say the clutch feels *heavier* now (which it shouldn't really if you have only replaced the seals), then the clutch must be operating normally, as with no pressure in the pipe to the slave the pedal should be much much lighter, which would also be the case if there were some air left in the system and the slave wasn't moving as far as it should.
P Hunt

Thanks for the replies,
I am sure the seals are the correct orientation, but if all else fails I will pull it apart to check.
The pedal is much lighter and I deffinately still have air in the system, despite pumping half a bottle of fluid through the system.
I spent last night reading manuals and books on the subject (If all else fails read the manual) and it looks like I was bleeding the system incorrectly. I was always taught to open the bleed valve on the down stroke and seal it off for the uppward stroke of the pedal (I think this should still work ?). All documentation states that with the bleed valve open you continualy pump until you get an air free flow.
Also when cleaning the system I found a "U" shaped piece of bent metal in the master cylinder reservoir, I assumed that this was some sort of clip which had fallen in from somwhere and had not been retrieved so it was not replaced and became one of those left over parts that you get from most jobs, is this piece of metal suppose to be in the reservoir to stop the splash back?
Somtime today I will use my new found knowledge and see if I can solve my problem.
D G Levy

Remove the slave cylinder from the trans and install a C clamp between the push rod and the back of the cylinder. Now, try bleeding it as you did before. These systems are notorious for trapping a small quanity of air in the slave making for a soft pedal. RAY
rjm RAY

Hi DJ,

I had the same problem trying to bleed my clutch. What I found is that once you have some fluid in the line then leaving it overnight it self bleeds. Somehow the fluid trickles into the slave and fills it.

H J Adler

The trouble with using the pedal is that the relatively large bore pipe in a long vertical run with an inverted U at the top traps air, which just goes up and down. But even continuous flow methods like EeziBleed doesn't seem to be the full answer. You have to be careful pushing the slave all the way into the piston because when you let it go again the internal spring usually pushes it out, and it can be easier to suck air backwards past the seal as it is to pull fluid all the way down from the master, which is why vacuum methods on the slave nipple don't always work either.

The easiest way is to connect the left caliper to the slave (same nipple size), open both then use the brake pedal *slowly* to fill/bleed the clutch. If already filled then drain or siphon some out first to prevent overflow, and watch the level in the small metal brake masters, although you should get enough through to partially fill an empty clutch master before getting near emptying the brake. Finally top off as normal.
P Hunt

Thanks to all for the help.
The problem has been solved and the lessons learned are as follows -
Open the bleed valve of the slave at least 3/4 of a turn (As stated in the manual) this ensures that the plastic hose fitted to the bleed valve is filled with fluid. It appears that my problem was that I was olnly opening the bleed valve 1/4 of a turn this would only allow a relatively small amount of fluid into the bleed tube and it never filled up and as a consequence air would be drawn back into the system.
The procedure of continualy pumping the pedal (As stated in the manual) was more effective than opening the valve on the down stroke and closing it off on the up stroke of the pedal, again one stroke of the pedal was not enough to fill the bleed tube with the bleed valve opened 1/4 of a turn.
I am surprised at the different feel in the clutch with the new seals fitted which is noticably harder, bearing in mind that the clutch always worked the only problem was a leaking seal which caused fluid to drip down the pedal onto my mat.
D G Levy

This thread was discussed between 13/12/2009 and 15/12/2009

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