Brake slave cylinder

[Sdinse]

I'm trying to find a rebuild kit for the brake slave cylinder. Apparently, mine has gone bad since the master reservoir is losing fluid and the slave reservoir is overflowing. So far, all I see are the complete brake servo repair kits at about $135. I don't really want to rebuild the actual servo itself, just the cylinder, so does anyone know of a rebuild kit for just the slave cylinder?

[MarekH]

Move the booster reservoir up to the same height level as the master reservoir. That'll stop you losing all of your fluid. Instead, you'll slowly transfer from one to the other as a small amount fluid gets past the seal on the booster shuttle every time you use the brakes. It's a glaring oversight on the part of the designers that they placed reservoirs at different heights.

If the bore is scored then a new seal isn't the only problem to solve.

kind regards
Marek

[Sdinse]

Thanks Marek, you always have good advice. I didn't have this problem before, but the car has been sitting for quite a while and so I'm thinking that a bit of use may return operation back to normal. At least, that is my first plan of action, otherwise I'll have to jump in with both feet and pull out the servo assembly, something I don't look forward to.

[Tommd]

It's a glaring oversight on the part of the designers that they placed reservoirs at different heights.

kind regards
Marek

Marek, not sure I follow. It seems if the slave seals are leaking, one would want to know so the problem can be resolved. Why would this being an indication be a glaring oversight?
Tom

[MarekH]

The problem you have is that the seal on the shuttle in the brake booster is not sealing any more. This is either because the bore of the brake booster s scored and/or because the seal between the two circuits has failed. Every time you use the brakes, a small amount of fluid transfers from the master circuit to the slave circuit. The level of brake fluid in the booster reservoir thus keeps going up and anytime the two circuits are open to each other, brake fluid just gravity feeds from high to low. The booster reservoir is mounted at a lower level than the master reservoir, so you lose fluid because it overfills and overflows the booster reservoir.

If the the booster reservoir were to sit at the same height as the master reservoir, then the propensity for liquid to go from A to B would be much diminished because a failed seal between the two circuits simply means you have one circuit, gravity fed by two reservoirs. What the result of that is is that the only a small amount of fluid is pumped from the master to slave circuit when you operate the brakes, rather than a gravity fed open circuit which empties the master reservoir very quickly.

You do have a problem to fix, but you may find it is only a small problem. If the reservoirs are mounted at the same height, you can't lose any fluid if you have a very bad seal, as the liquid always finds its own level, so the reservoirs equilibrate to the same liquid height. If you only have a small leak, which manifests only when you are pressing the brake pedal, then you can simply syringe the excess found in the booster reservoir back to the master every 100-150 miles or so and that buys you all the time in the world you need to replace the leaking booster shuttle seal.

With the reservoirs at different heights, liquid always ends up in the lowest reservoir. That's why you are losing liquid - the lower reservoir overflows because it is too low compared to the upper reservoir. The syphon effect empties the top reservoir.

With the reservoirs at the same height, the same syphoning which is emptying your master cylinder now aids you - it tries to make sure the the liquid in both reservoirs transfers until their levels are the same.

You have only two basic scenarios:-
1/ the seal is badly compromised (i.e. the seal itself is now bad or the bore is badly scored) and liquid just gravity feeds down and you lose lots of brake fluid quickly and always;
2/ the sealing is bad somewhere along the bore but is good at the default seal resting position. Only a small amount of liquid transfers from A to B every time you brake.

In either case, you are better off with both reservoirs at the same height. With 2/ you can just syringe the excess back to the master between trips. With 1/, you only have one brake circuit anyway now.

In neither case is there any reason to lose brake fluid - the problem is internal.

kind regards
Marek


kind regards
Marek

[Tommd]

Marek, Sorry you had to write so much. I understood what is going on and why- the fluid exchange is quite obvious. That is not what I was asking. I was commenting that if anything it may be good that the fluid levels are changing as that helps to alert one to the fact that a seal is leaking. Although I am not saying I always fix things immediately, I contend this seal should be fixed ASAP. This seal is part of the dual master cylinder setup. One would not want to find out about a failed seal in one system as a result of a failed seal in the other system and thus having limited to no brakes.
It sounds like you may not agree, but I see this "flaw" as an advantage with not downside. It helps to motivate one to fix something that could be serious.
Tom

[Sdinse]

UPDATE!

After working the brake 15 to maybe 20 times in the garage then going for a few 5 mile stop n go trips, checking levels each time before & after, it seems to be back to normal operation.

I was really surprised there were any problems with this thing since it was new about 10 years ago, filled with DOT 5 silicone fluid and has always worked perfectly until I checked a few days ago. It had not been used for quite some time as life had interceded and left me NO time for the Jag for maybe 3 years. In the last 10 years I've probably only put 1500 mile on it, so I don't think I wore it out. I'm thinking the seals just needed to get seated again. I think it's as Marek said, gravity is the enemy.

[mgcjag]

Dot 5 silicone is not a fill and forget product....standard fluid is typically replace every 2 years.....silicon that I use quotes 8 years but I replace at about 4 years....Steve

[MarekH]

It sounds like you may not agree, but I see this "flaw" as an advantage with not downside. It helps to motivate one to fix something that could be serious.

My suggestion lets you still see the problem, only it won't lead to an immediate and catastrophic leak as well.

With reservoirs at the same heights as each other, the failure mode becomes "the level in the master reservoir seems to go down a bit and the level in the slave reservoir go up up a bit!" 100 or so miles after seal failure, instead of "oh shit, there no fluid in the master reservoir and the slave has totally overflowed and blistered the paint on the inner left hand wheel well".

It still needs fixing, but all my suggestion does is give you a better failure mode going forwards, not an excuse not to fix it.

The way to decide whether you have a problem is next time you bleed the brakes, to pack or dissolve lots of red grease into your dot4 fluid which you use for the master reservoir refill, but use straight (uncoloured) dot4 for the slave reservoir. Bleed the brakes normally. If you can see a whiff of red come through in the slave circuit as you bleed, then fluid must have transferred between them. If the slave circuit remains straw coloured, the seal was good.

kind regards
Marek

[Tommd]

Again, thanks for your thoughts. I guess we just have different opinions of "glaring oversights."
Tom