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MG MGB Technical - bleeding hydraulics with eezibleed
|I've got to the stage where I have to bleed the brakes and clutch. I did the brakes with a pipe and non return valve method (my attractive helper was not available) and that seemed to do the trick. Doing the clutch was not so easy that way so I got one of the pressurised Eesibleed devices which seemed to work quite well. However the pedals very soon became 'soft' (both brakes and clutch). Are there 'secret ways' when using the eesibleed?! I have one of those replacement clutch master cylinders which have a very small plastic reservoir which hardly hold any fluid at all. Using the eesibleed means I dont have to top it up after every pedal pump! As far as the clutch is concerned is it best if I push the push-rod right into the cylinder? Do I need to remove the slave cylinder and have the bleed nipple uppermost? I'll give things another try tonight when I get some more brake fluid.|
|What do you mean the clutch pedal has gone 'soft'? Unlike the brake pedal the clutch is *supposed* to go all the way to the floor. If you can engage reverse with the engine running without grinding then the clutch is fine, it could well have just 'bedded in' which was something I noticed on my V8 after replacing both cylinders and the flex hose.|
If reverse *is* grinding, and it wasn't to start with i.e. when you first started driving after bleeding last time, then there sounds like there is something wrong somewhere. If you are not losing fluid then either air has got in somewhere, or the master pressure seal is leaking back. Check the travel of the slave push-rod, which should be about 1/2". If less than that the first step is obviously to rebleed, at least to confirm what is happening.
Chris Betson is adamant that clutch bleeding is easy and gave his method on another thread recently, but I've been reading of the problems people have here and elsewhere for years. I've never even bothered trying to bleed top-down with either pedal or EeziBleed. I did reverse bleed with the EeziBleed but since then an easier method came my way which is to interconnect the clutch and right caliper bleed nipples, open the clutch nipple, apply slight pressure to the brake, open the brake nipple, and slowly and gently push the brake pedal to the floor. Close the brake nipple before the up-stroke. A couple of pumps should be all that is needed to push any air back to the clutch master, which will need some draining out first if it is full. A couple of pumps shouldn't lower the brake master significantly, but keep an eye on it, and top-up afterwards.
I'm not a fan of pushing the slave piston all the way back in, there is a spring inside which pushes it back out again, giving negative pressure inside the cylinder, which can suck air in past the seal *causing* the problem - BT, DT. Make sure you have the bleed nipple higher than the flex hose, slave cylinders are usually shipped with the nipple in the other port. Unless you have the rear of the car a lot higher than the front for some reason when bleeding the passageway from the cylinder to the bleed nipple is at the highest point for *normal* i.e. top-down bleeding so can't trap any air. Even though this puts the outlet i.e, the flex hose for reverse bleeding in the lower position, and hence is potentially able to trap air in the cylinder, I've not had any problem with reverse bleeding and it only takes minutes.
Again with brakes, if they *were* all right initially, but are now spongy and long but can be pumped up with a couple of quick strokes, only to go long and spongy again a few seconds later, then again there is air in them which indicates there is a problem. With MGBs I've always had to bleed in two stages - first fill the system and get apparently all the air out using the EeziBleed conventionally, but that always leaves them spongy as I have described. The second stage is to have someone press down as hard as they can on the brake pedal, then you rapidly open and close each caliper nipple in turn, and this has always blasted an extra lump of air out. After that they have been fine.
|I've just had another go at bleeding both systems. The brake pedal does not become firm all the way down until it has been pressed a couple of times. I assume this means air still in the system. The clutch slave piston has maybe 1 cm of travel and does not start to move until the pedal has travelled an inch or so. I notice that there is about 4mm of travel at the master cylinder pushrod before I get any pressure. Is this normal?|
Both systems are new apart from the hard brake lines. Master cylinders are the ones with plastic fluid reservoirs.
I'm going to try using an attractive assistant to get some real pressure behind the pedals to try an force any air out.
One other thing is that the brake pedal does not return very strongly. Is there supposed to be a spring somewhere in the system other than the one in the master cylinder?
|Brake pedal seems better now after pumping with the pedal. Clutch slave rod had 8mm travel! Using same technique it was getting better until the reservoir ran out of fluid and air got back in again. Now need to get more fluid. These replacement clutch MC only hold enough for two or three pumps of the pedal! Will try again tomorrow evening.|
|There should be a strong spring with one end attached to the pedal and the other to the underside of the pedal box panel. i.e. just above where your toes are when pushing the pedals.|
Similarly, there should be one attached to the clutch pedal. The part number from Moss is AAA1628 for both.
|Both master cylinders have some movement before any pressure starts to develop, this is because the pressure seal has to close off the bypass port from the reservoir into the cylinder. Until this happens fluid is simply pushed up into the reservoir.|
You still don't say whether or not you can select reverse without grinding when the engine is running. That is really the only criteria that matters. 'maybe 1cm of travel' is a bit imprecise but if true is not enough if it *is* grinding. You can also get low slave piston travel from wear in the linkages up at the pedal i.e. the pedal hole, master push-rod holes wear oval and the clevis pin gets wear grooves. Without the pedal spring the master spring should push the pedal nearly all the way back, with just a little free play. The more free play you have the more wear is indicated in the linkage. Note that the similar linkage at the slave end is *not* a factor, the self-adjustment feature of the hydraulic system copes with any and all mechanical wear at the clutch end.
Brakes "better now after pumping with the pedal" is not good! Unless they retain it long term, or unless you mean 'bleeding' with the pedal.
Both pedals should have those return springs. I've had the clutch one snap twice, it seems to have too much tension it it at rest to me hence is being stretched too much, so I've stretched it out a bit so it just pulls the pedal back all the way (but doesn't rattle). It also seems to be bent as the pedal is operated if fitted the easiest way, it is that which has caused it to snap I reckon. Fitted the slightly more awkward way it doesn't seem to do that.
|Last week my throw out bearing blew apart, so I replaced the cluth. In the process I've been repairing all sorts of other items noticed during the engine removal and replacement. One of the items was the need to replace the hose to the slave cylinder on the clutch (as well as any bushing, rod, sleeve, etc. to reduce play in the system).|
We spent a good hour plus, pumping - bleeding - pumping - bleeding - but couldn't get the air to move through the system, we couldn't even get fluid to move through, until we borrowed a pressure system. I have no idea what the system was called.
We never tried the process suggested above utilizing a reverse flow and the brake caliper.
We are assuming the air was trapped in the tube that rises above the clutch master cylinder before it decends to the clutch. We tapped on the line lightly during the bleeding, and bubbles came through almost in sync with the tapping.
It now passes the shifting into reverse test, but boy is the new clutch soft compared to what was in there.
I didn't have these problems bleeding when I replaced the master cylinders a couple years ago.
|Paul, thanks for the comments. What I meant by "better now after pumping with the pedal" was that I used the eezibleed to get fluid into the system but had to use the technique you mentioned at the end of one of your replies where the pedal was pressed rapidly when the bleed nipple was opened to remove any trapped air in the slave cylinders. This has produced a much firmer and consistant pedal for the brakes. I also had to do the same for the clutch. I now have about 11mm of travel at the slave. I have only just got the engine running consistantly having just set the static timing and the carbs to their '12 flats down' position so engaging gears with the engine running has not been possible. I'll try that tomorrow. |
I can see that there have been springs fitted in the past but they are now nowhere to be seen so presumably the PO removed them for some reason. As the pedal cover was also removed I can only suppose that he was working on this area at some time in the past.
|Steve, just focusing on the brake for a moment, do you have a servo and a re you bleeding the brakes with the engine off?|
Feel free to come back and say "I am not that stupid".
|David, no I don't have a servo. As far as being stupid, anyone who believes they know everything is doomed, doomed I say.|
|Steve. on side note I find these things great for Bleeding, make life so easy|
|I've never used Speedbleeders (too jolly expensive for something that's only going to be used once in a blue moon) and they are superfluous with the EeziBleed. But neither are going to be any help with the second-stage 'high-pressure' bleeding that I have always found necessary. There has been a suggestion that this stage is needed *because* of the remote servo, which on the MGB seems to be incorrectly orientated according to the instructions that come with that servo when used as an add-on. These show the valve angled downwards instead of upwards, and the servo cylinder angled markedly upwards towards the outlet instead of pretty-much horizontal. Not having bled an MGB without servo I don't know.|
|I would like to echo PaulH's tip about secondary bleeding, especially brakes. I recently had to replace the rear brake cylinders on my 72 roadster and did the "attractive assistant with foot hard on the brake pedal" trick and it worked a treat! When I fitted the cylinders I noticed that the bleed nipples were BELOW the feed lines!!! This means that there will be air trapped in the cylinder so a quick full pressure bleed will force the air out(hopefully!). My brake pedal is now nice and firm, first push. Thanks Paul, us ex-TO's have to stick together.|
|(That'll get people wondering - "PO is Previous Owner, but what the hell is 'TO'?")|
Unlike calipers it shouldn't matter that the rear cylinders have the bleed nipple below the supply port as unlike the calipers the slave pistons are pulled back in by the springs when the brakes are released. When they are pulled all the way back in the two are back to back and there is only a narrow channel round them from supply port to bleed port, so air can't get trapped any more than it can in a length of pipe. Whereas in a caliper the pistons are inching out all the time as the pads wear, and the cavity behind them is getting bigger and bigger. However slave pistons are only pulled *fully* back in when the handbrake cable is disconnected, as ordinarily it is the handbrake levers in the drum that are holding the shoes out part way, hence the pistons don't get pushed fully back in. But in 40 years of bleeding BL brake systems (take that either way) I've never found that a problem.
This thread was discussed between 11/10/2010 and 25/10/2010
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