1984 JCB 3CX rear Brakes

[bdfi69]

Hi to everyone from bonny Scotland & hope everyone coping with lockdown. This site is amazing & I have used it plenty to expand my knowledge to keep my digger going, but alas I am stuck & need some advice on the following if anyone knows please. My 1984 3cx extradig is still going strong, but I have a little issue with one of my rear brakes, one works perfect, the other side only works when you pump the pedal 3 times, its like there is air in the system, but no air as I have bled several times, plus I also replaced the master cylinder with new one & still the same issue exists, so after browsing all over the site here, I learned that I need to change the fluid out as my brakes have Dot fluid in them, so I will do that this week with HP-15 & I will also buy 10-30 gear oil to change out the axel oil as not sure when it was, if ever changed.

So my question here for help is, I guess there is a seal inside the axel area somewhere that may have failed or it may be something else, as when pedal pumped 3 times, the brake works really good, just feels like it's not holding pressure & allowing fluid to maybe bleed back, I have no loss of any fluid from the brake fluid reservoir so not leaking, any ideas welcome! also can this be an easy enough job to repair or change out for the likes of me with a fair bit of mechanical knowledge? I will also take some pics of the job as I go along, if it's not like a full axel off & strip down 2 year job ha ha. Thanks for any advice in advance.
Added in a pic of my beast & what I got from JCB on line parts which my axel looks the same, however not sure.

[essexpete]

It might be that the DoT fluid has scuppered seals in the system.

[bdfi69]

Well after reading loads on here, I was begining to think it must be something kinda simple as the brakes still work if you pump the pedal a few times, but I have no real drawings or that to look and see & really assuming that it would be a relatively simple replace of a seal or two, time consuming maybe but easy.

[essexpete]

Never did the brakes on our JCB but they ought to be a more easily accomplished job than many typical agricultural tractors where the whole axle trumpet has to be removed.
When you replace the oil make sure you use the correct spec gear oil. I always had a job to stop brake squark or chatter on our JCB.

[AJ.]

More than likely the problem is the master cylinders,they may not be leaking,what happens is the pistons do not return fully,when that happens the cylinder does not get a full charge of fluid and the travel of the piston when the pedal is pressed is shorter even though the pedal goes lower,to test if you have the gear pressurise the system by the bleed nipple,ie you are pushing the fluid back which in turn will push the pistons back and you will have a good pedal.
AJ