Talk Morgan Forums Morgan Sports Cars Morgan Photo Gallery BTR Axle Breather

Morning all!
Recently had a quick chat with Dave W, re. the rear axle breather on my 2010 plus 4. I was in the process or changing all fluids on the car when I got to the rear diff. I cracked open the filler plug prior to emptying the oil to make sure I was able to refill it after I'd drained it out! Then I started the drain process. On undoing the drain plug which is incidentally the central lowest bolt on the Diff cover, As I undid the bolt at about half way removed I got that tell tale hiss of escaping air which was clearly under pressure and clearly too much of it as well!
I knew straight away that this pointed to a problem with the axle breather. I always keep an eye on the breather cap, making sure that it is loose enough to move around easily but something was clearly amiss. So this morning I raised the car on the lift and using an old toothbrush cleaned around the area. I then used a 7/16" AF open ended spanner to undo the breather ( breather cap easily moves up to accommodate this). I than took my time in undoing the breather moving the spanner back and forth to gain removal, somewhat akin to tapping a new hole; remember, the breather has a hole right through it's centre and could easily shear if to much torque is applied!
When out and cleaned up a little I used the old mechanics technique of blowing through it and it clearly wasn't functioning correctly. So I shot some penetrating oil through the valve and eventually things started to free up. I then used the airline to clear under the cap and carefully to blow through the valve. From the pictures you will see that it has a BSP thread as I was able to remove the air gun nozzle and screw the breather in where it had been. Now using the mechanic's lips again I was able to ascertain that the valve was functioning correctly. The dust cap should be under tension from a spring and that spring also holds down a ball acting as a simple valve onto the central hole on the breather. So in effect the ball produces a valve type function. this does require a small amount Of pressure for the valve to open but clearly shows that the axle will have some level of pressure when hot and working. There are two types of valve, those which function in a passive style, ie. no resistance at all, sometimes just an open drilling in the axle tube which also acts as a means of correcting the oil level and these are notorious at blocking up, all the way to complex pressure relief systems and sometimes raised breathers for off road and agricultural vehicles. What's the importance of the valve or open drilling working? Well, too high a pressure within the axle casing can force the oil past the outer axle oil seals and will usually show as contamination on the brake back plate. When this happens it will usually require a full hub strip down to replace the seals. You'd be surprised how often this job is undertaken only to shortly reoccur as the initial problem had not been addressed (the blocked breather). This job, done as a check can be quickly undertaken by jacking and supporting the rear of the car and removing the off-side wheel for easy access.

Good find and well corrected Given the retained pressure you were lucky not to already have diff oil escaping.

Just noticed on re reading your post that the pressure escaping was when drain plug was removed. Surely it would have happened when filler plug was first removed

Hi Richard,
Yes i see that but no it wasn't ! I never remove the filling plug fully first (loosen only) as that first gush of oil on removing the drain soaks the fuel tank, your hand and also in the case of the mog soaks under the planking the tank sits on. I too would have expected it to have pushed the oil out but no, not a trace of oil just air! then when I removed the plug out came the oil as expected. The oil was cold as I hadn't run the car first as I'm also sorting the clutch hydraulics' as well. I was too surprised that the seals had tolerated that pressure, way beyond anything I had seen before.

When I was talking to Dave W I did also query vacuum to explain the air movement noise but having seen the valve type the design is to prevent ingress of water by sudden cooling if fording (hot diff into cold water) or in the case of the Morgan should I say 'submariner training'! and this valve was clearly locked shut, remarkably so from new probably by the sprayed rust protection shooting up under the cap which had clearly been sprayed as you can see in the picture prior to removal. Although I wouldn't rule out the possibility that this had happened through a cooling event. It would also have the effect of increasing the seal of the breather especially as it had a wax seal keeping the ball bearing securely in place. Think perhaps I'm coming around to this being the theory solution here Richard as I just can't see how the oil wouldn't have been forced out of the threads whereas air could enter under vacuum with just the sound of air, no liquid bubbling etc.

In truth, I can't fully explain it. I only just cracked the filler to ensure its removability and there was no air movement at that time. So it must have been a vacuum pulling air into the axle casing? The amount of air moved was fairly substantial and were it pressure I feel sure the seals would have failed. This can't be a first so perhaps there's a boffin on diff's out there who can answer this riddle! shame HOW is not on the tele anymore!
I can't know the history of this car because my family are not original owner's and with low milage the servicing was spasmodic at best! I think the gearbox and diff had never been serviced but the engine oil three times at best. brake fluid changed but not clutch. Probably frightened off by the concentric clutch.

Thanks for explanation sewin. Typically I would remove filler plug completely then put it back in with a couple of turns to control flow from drain plug. That would have fully released any pressure either way first. I always change oils when hot or at least warm, it comes out easier then and any debris is likely still in suspension. Gloves stop it covering your hands.

The best you can manage with a concentric clutch slave and no bleed nipple, as on my MT82 gearbox, is to suck out most of the fluid in the master cylinder without allowing air in, then topping up.

Yep I always go for hot oil too, this is the first time for me changing cold -anything oil! I'll change it out again in about 6 months!
My MT-75 gearbox has a bleed nipple although I have to agree with the reservoir change technique as the concentric slave is just full of problems and fragile when it comes to pressure bleeders. I always reverse bleed if I have too ( using a manual syringe) but if you change the full reservoir every year it seems to be fine. However if you stuff the slave it's a gearbox out job! Mind you Ford used a traditional external slave when using this box!