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Repairing a Brake Master Cylinder

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The master cylinder I have used in this post was originally on a TD 90. It's going on catflap, and was fine when it was taken off the donor vehicle, but it's been stood for about a year, so I thought it wise to play it safe and replace the seals.

This job might seem daunting, as the braking system working correctly is obviously vital, but the tools needed are minimal, and concentration are all that's needed to do this.

Bear in mind that the components are very delicate, and new parts MUST be put back exactly the same way as the old ones came off. Some of the seals may look the same, but on closer inspection they are not. A clear work area, no distractions, and a vice are the most important.

Tools required are:-

Long-nosed pliers

Internal circlip pliers

Small hammer or preferably mallet

5mm drift (or blunt 5-inch nail)

Thin electrical or similar screwdriver.

Other items:-

Fresh supply of DOT4 brake fluid

Lint Free cloths

The Master cylinder - genuine Lucas unit.


There are two options - one is a seal kit from the likes of Bearmach, and will set you back about £11, or there's the genuine overhaul kit, which was £21.


The difference in the contents. The one on the right has no instructions or anything other than what's really a bag of seals. Ok if you know what you're doing, but looks cheap and nasty.

The overhaul kit is comprehensive and has more components. It has an instruction sheet in it, the seals are in a seperate packet, just more professional-looking really. This is the one I will be using.


On with dismantling:- The reservoir is held in place by two 5mm roll pins. Tap them through and out the other side with a 5mm drift or blunt nail. Carefully wiggle the master cylinder and it will come away fairly easily. Too much force and the two stubby pipes on the bottom of the reservoir could break or split.


The two rubbers that are now exposed can be gripped with the pliers and pulled out. There are new ones in the kit, so discard these.

This small pin is almost all that really holds the internal components inside the body of the master cylinder, leave it in place for now.


Place the reservoir body in a vice, clean the end, then remove the nylon collar with the circlip pliers.


The main piston assembly, only now retained by a small pin.


The pin is held in place by tension of the spring in the first piston assembly. To remove the pin - use a stubby screwdriver or similar tool to push the plunger back inside the body of the master cylinder only a few mm.


While maintaining pressure on the piston, remove the pin with the pliers. It'll come out very easily.


Once pressure is released on the piston, spring pressure will push it part way out of the bore.


There's another piston assembly still inside the master cylinder. Rapping the master against a hard object will move it outwards.


The entire internal components of the master cylinder.


Clean the inside of the bore until it's spotless. Don't use anything to clean it other than clean cloths. Look very closely for score marks, or a step where a piston may have been digging in. A master cylinder with a badly scored or stepped bore is no good and a new unit should be fitted. The brakes might be fine for a while, but it's likely they will start to fail sooner rather than later.

This is perfectly ok.


The old components at the bottom of the picture, and the new parts directly above.

The two seals on the left that have to go on the inner piston are different from each other, make sure the right one goes in the right place and the right way round.


The inner piston dismantled, The spring has a tapered end, and is a tight fit, but will twist and pull off. The seal retainer, seal, and washer will then just slide off. The seal at the other end is in a recess, so a thin screwdriver or similar tool is needed to remove it. Pay attention to how it all comes apart, and put it back together the exact same way. You'll need to use the thin screwdriver to remove the right hand seal, so be careful not to cause any damage to the piston.


Once assembled, lubricate the bore and new seals with fresh brake fluid, replace the inner piston with the spring in first, lubricate the outer piston seals with brake fluid and apply pressure again with the stubby screwdriver, until the whole assembly is back inside. Be very careful doing this, as the new seals go into the bore, the lip of the bore will try to turn the seal. Keep an eye on each seal as it goes in.Then put the new pin in the hole and slowly release pressure with the screwdriver. If the pin has located correctly, the outer piston should stay inside the bore.

Using the circlip pliers again - put the new nylon collar in.

The two seals that go on top of the master cylinder are a very soft type of rubber, so I recommend not to use any tools to put the new ones in. Just coat them in brake fluid, push one edge in like this.


Then apply pressure with your thumb and the seal will pop in and square itself up at the same time.


Make sure the two seals are still wet with brake fluid and press the master cylinder back in place, so that the two short pipes on the bottom of the reservoir are inside the two seals. It's possible to put the reservoir on back to front, so make sure the highest part of the reservoir is at the fixing holes end of the assembly.

The reservoir is held in place by the two roll pins. Carefully tap the pins back through so that approx 4mm sticks through on both sides.


Take care doing this, the tabs on the reservoir could easily be broken off by the misalignment of the roll pins.

Wipe off excess fuid from the master cylinder, and the jobs done. The Master now looks almost as good as new, and it's performance should be similar. Re-fit to the vehicle, bleed the system using fresh fluid, and that's it.


I can't stress enough the care needed when doing this job. If a seal is in the wrong place or the wrong way round, you may not know it until your brakes fail or under perform.

Also, a worn seal doesn't look worn, and can be confused with the new one lying next to it. Once you have identified the new seal, and it's position, chuck the old one away.

Les. :)

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