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MG MGB Technical - can't engage reverse gear
|I'm new to this MGB board so hello to all members!|
usually I'm dealing with my MGA only but with my friends 1976 MGB GT with overdrive we got problems. The reverse gear won't engage, we checked the overdrive solenoid, it's OK, changed the slave cylinder, bleeded, no difference. If we operate the clutch release lever with a bar the gear willslip in with no crutching, we think the new slave cylinder just does not push out the rod far enough. Anyone knows the measurement how far this rod is normally pushed out? Ours reaches only 10 millimeter.
|Does the gear grate when you try to engage it as this would indicate a clutch withdrawal problem?? From memory the clutch fork moves about 13 - 16 mm so 10 mm would seem a bit short and could well be the reason for your problem|
yes it does grate, all forward gears engage fine - did not go on the road, just had the car lifted. We reach 16 mm only if we operate the lever manually, so we'll do the clutch master cyl next.
Have you installed the bleeder nipple on the top (upper) outlet on the slave cylinder? It sounds a lot like you still have air in the system.
|Travel should be about 1/2" to 5/8" or 13-16mm as Iain says. If you push the pedal right down and engage 1st, and find that even though you are still holding the pedal down the car starts to creep forward i.e. the clutch is engaging on its own, then there is a hydraulic leak, If this is not visible on any external part of the master, slave or pipework then the master seals are worn. If the clutch *doesn't* progressively engage with the clutch pedal held fully down in gear, then I doubt the master is at fault.|
One cause of low slave travel and hence clutch dragging and a low biting point is wear in the mechanical linkages between pedal and master push-rod. Ovalling of my pedal and push-rod plus wear on the clevis pin meant that my clutch pedal had to be about an inch lower than it should be. However this wear occurs gradually so if the problem has suddenly developed it is unlikely to be that. Note that wear in the mechanical linkages at the slave end is *not* an issue, the hydraulic system is designed to take up all and any wear that occurs that end, as the friction plate and more importantly the release bearing wears down.
The other cause of low travel is of course when there is air in the system. Bleeding is problematic on the MGB, so many say. Personally I've not had a problem as I always bleed from the bottom up, which is the natural way the air wants to go. The easiest way I've found of doing this is to connect a tube between the slave nipple and right-hand caliper nipple, syphon some fluid out of the clutch master, open both nipples, then use the brake pedal *gently and slowly* to push brake fluid backwards through the clutch system so taking any air with it. You have to remember by connecting an empty tube you are going to introduce some air anyway, unless you pre-fill it from either clutch or brake. I'd suggest the clutch to avoid introducing air into the brake system when their wasn't any there previously!
there are differnt qualities of 'look alike' slave cylinders on offer in Germany. May be that the issue is forced by that.
An other point to check are the rods and the celvis pins at both of the cylinders as they can wear our dramatically, robbing the lift you need.
I think the question is that interesting, you should also post it on http://www.mgdc.de in the drivers talk.
|Good point about the bleed nipple position, mine and Conrad's posts crossed in the ether as it were.|
They always seem to be shipped with the bleed nipple in the wrong position, either because the slave was originally specced for another application that needed it there, or they save a groat in packaging. Any road up, when fitted the hose port should point to the front of the car, and the bleed port across the car, which does put it at the top as Conrad says.
And just to repeat, linkage and clevis pin wear at the *slave* end does *not* result in short travel of the push-rod. Not only are they after it in the overall system, but the hydraulic system compensates for all mechanical wear at that end.
|thanks for all that valuable information!|
re bleeding, removed the slave and pushed the piston fully back and bleeded. After assembling there is no air in the system.
re master cylinder, most probably there lies the fault as rod, pin and pedal are a bit on the loose side, will take care about that, also a repair kit will be fitted.
can't move the car on the road as it sits forward in the garage and the drive is quite steep. Will let you know when we have the problem solved!
|The push-rod, clevis pin and pedal *will* be loose with respect to each other at the pedal spring pulls the pedal back further than the master spring pushes it. If the master piston comes back as far as the circlip then the master itself is fine, but you could still have *excessive* play between the pedal and the master push-rod. If the master push-rod isn't coming back far enough then its seal won't clear the passage leading from the reservoir and you wouldn't have been able to push the piston back in the cylinder and hence push fluid back through the system into the reservoir, unless there is air still in the system anyway. |
How do you know there is no air in the system? As far as pushing the piston back goes there is a spring and plastic disc behind the rubber seal, which is cupped on the back face anyway, so you can't push the piston right to the back of the cylinder and hence push all the fluid and any air out of the cylinder that way. Also the hose and bleed nipple passages into the cylinder are in different places - only the bleed nipple passage is right at the top and across the angle between the back and the cylinder walls, the hose passage is part way down one side of the back, leaving space for air above it. Pushing the piston back *is* useful for pushing any air that has gathered in the loop at the top of the pipe leading to the master, back into the master reservoir and hence out of the system.
|Have you tried "pumping up" the clutch? It has worked for me. Might be enough to get it into reverse. Also putting it in another gear first to stop the gears turning.|
This thread was discussed between 15/01/2009 and 18/01/2009
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