Brakes on Massey Ferguson 390 tractor are operated hydraulically, when the 500 series MF brakes are operated by iron bar linkage. My 390 had started to loose brake fluid and left brake didn't work well, so some left side brake cylinder apparently leaked. In early 300 models to serial number V39466 (1987) the cylinders looked the same as in the 600 series. This 390 has newer type slave cylinders, that are inside the rear axle housing. When the slave cylinder leaks, the fluid gets among transmission oil. For this reason 300 series tractors use mineral oil based green brake fluid (LHM). I myself had added ordinary 10W-30 transmission oil to brake fluid reservoir and the brakes worked fine with it too. The brake discs are in transmission oil, so there isn't need for high temperature tolerance of DOT4 style brake fluids. Still 600 series MF have DOT4 type brake fluid and the rubber seals are chosen for this type. In 300 series it's even more important to avoid wrong kind of fluid, cause it can leak to transmission housing and make damage there.
The handbrake is wire operated and usable even if the fluids disappear. The letter starting the serial number tells the apr. manufacturing year: U for 1986, V for 1987, N for 1988, P for 1989, R for 1990, S for 1991, A for 1992 and B for 1993.
The brake fluid reservoir is in cabin behind the right side panel. The order to use mineral based fluid is written also on the side of reservoir. I gave about 1 bar pressure in the reservoir through that rubber hose and tyre filling valve to make the bleeding easier, but must be careful with that, cause old plastic can be fragile.
These master cylinders started to leak too and some oil came inside the cabin and made me to start the repair. I ordered only cheap sealing kits, cause I was unsure of the proper model of the whole cylinder.
That actuator rod on the left can be twisted away from the blocks on the brake pedals using a small wrench. Before this open the locking nut, it's not in the pic. Now the pipe nuts can be opened and the cylinders removed. This cylinder is 15,8 mm of diameter.
The piston parts are in wrong orientation in this pic. I fastened the piston to vice and stretched the greased rubber seal carefully on it using mostly fingers. You must bend the little tongue on the spring retainer cup to take the valve assembly to pieces.
I put that first cylinder together, but then found out that the other was with a broken spring. So I ordered two repair kits, that contain whole piston, spring and valve assembly. At first aimed to buy the whole cylinder sets, but couldn't find 2 of them right away and the old ones didn't look too bad either, no corrosion etc.
So now I have one brake master cylinder with new spring and the other on front with the old spring, cause it was already assembled together. The fluid was still vanishing, though it didn't leak in the cabin any more.
PS: That first cylinder started to leak again rather soon and I suspect that green seal kit to be bad quality. Must try again with this repair kit in the pic.
Next I took the left rear wheel off and started to dig my way towards the brake parts. At first had to take some hydraulic lines away, some of these are for the front end loader.
The handbrake wire can be separated by loosening the adjusting nuts and that spring can be forced off with a screwdriver. 300-series tractors onwards serial number V39466 have this kind of brake actuating slave cylinders. Before that cylinders are outside the rear axle housing like in 600-series.
Several hydraulic tank lines come to that cross shaped connector on left side round cover. There return oil is led inside the central housing.
On the background the safety switch that prevents starting when Power TakeOut (PTO) is on. Traditional way on MF the other safety switch is on divider gear stick, shortest of them three. On the front a hydraulic manifold to combine flows from linkage and auxiliary pumps, I had to take the manifold off. At this point I remembered that transmission oil level probably should be lowered before I remove the brake actuator. I wanted to get oil out clean and some of these hoses were already disconnected, so I let tractor's own pump to pump some oil out through one of these hoses. When pumping that oil, I found out some things about hydraulic system, so I wrote about it on my MF390 hydraulics page.
The handbrake eliminated and hydraulic manifold separated on brake shield.
The higher the rear axle and this side reduction planetary gear is jacked, the more oil can be left inside.
The slave brake cylinder, cover and parking brake actuator. And the new seal kit I bought years ago.
I tried to install the piston seal with the aid of a socket, didn't fancy it this way. Instead I fastened the piston to vice sideways and pulled the seal over the piston using my fingers and some screwdriver. Rubber seal stretches a lot, but can take it when greased.
The new seals on the pistons. The piston with hole must be lowest to get the air out. Rubber boot for the parking brake rod on the front.
The spring tries to push the pistons out and it's a bit hard to press those piston seals in the cylinder. So it's better to tie the pistons to right position. Also suction keeps the pistons in, if both the holes are closed. You could do this by borrowing the bleeding valve from the other side and placing it where the brake pipe should be. Maybe just finger might do the same long enough to install this whole package in rear axle. I didn't know this and installed first the plain cylinder and the cover after that. On the left the O-ring to jointing between cylinder and cover.
2 of 4 brake discs at sight. I like this brake design, cause the cylinders come out without removing the rear axle housings and the brakes work well. Inserting wrong kind of brake fluid is about the only risk; Of course the rubber seals get old at some point anyway. Iron bar linkage in 500-series brakes need adjusting every now and then, not so much repairing. Of course hoses are better choice for a cabin with better suspension, that moves more.
Pistons press the lugs and rotate expander plates on each other. Balls between plates push them against the brake discs. Handbrake actuator rod on the right.
The cylinder package stays there quite well without that paper filling too. The cover is best installed by looking on direction the wheel was. The cylinder can be installed with the cover, I'll try that on the right side brake, it's still undone. At least got this photo when doing this way. I used silicone sealant on the jointing and thread locker on bolts.
I had the hose end in a half full bottle to stop vented air return to cylinder when I pumped the pedals. The handbrake must be off and I also fed compressed air to the fluid container, see first pic. Leaking cylinder vented by itself every time I inserted new oil. Right side is still with old seals until they start leaking. Next I will drive around the yard and adjust to get the wheels to brake even. Brakes are among the things you learn to appreciate, when they are missing.