I happen to have a MF50 and it is very much like the 135 with quite a few exceptions. However the lift systems are very similar. Drain I believe its 3 gallons of fluid, it should hold 8. Take off either the pto lever cover on the left side of the tractor, or the dipstick side, preferably the left side with the pto lever because you can chance the filter if you have one, though the early MF50's from what I read actually did not have this filter (1957-early 58?) Then with the tractor running and a slight load on the 3 point hitch, have it lift w.e it is, and take a flashlight and shine it up through the hole, and look for a lot of fluid leaking from above, it if is, you would most likely have to rebuild the lift cylinder, I actually have to do it on my MF50 also, lucky me... BTW do not stick your hand inside the case with the engine running and other various part moving. And keep in mind it will cost approximately $60-$90 to fill the tractor with hydraulic oil because it takes 8 gallons! Probably either 80w mineral oil (winter) and 90w (summer) depending on your operation, this can be found at NAPA, your tractor dealer, or possibly tractor supply. Check the fill plug underneath and the the left of the steering wheel, in front of the gear shifter. Also, if they are cracked and dry rotted, I would replace them as this is an area where water likes to get into the hydraulic oil and contaminate it. Do NOT fill with gear oil. Also, if planning to change the fluid, after draining all of the hydraulic oil out, let the tractor sit for a couple of days with the drain plugs out because it will continue to ooze out for awhile.
Btw it could also be a relief valve, I believe it cost around $20, it could be that this was blown at one point or got stuck open because someone decided to try and lift something considerably heavy with the three point in the normal position, but wasn't enough so they raised the draft up all of the way, and the relief valve blew... Ask me how I know...
Goodluck:thumbsup:,
Charles