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TR4/4A Early TR4 - Front Brake Calipers



I am trying to fit new pistons into new seals.

The manual says to:
- fit the seal into the bore - done.
- fit the dust cover lower edge into the caliper groove - aargh (not done).
- slide the piston through the dust cover and past the bore seal into the caliper - tried this first and it won't get past the seal even with liberal amounts of new fluid.

What is the trick in getting the dust cover in place and then managing to keep it there while I try and fight the piston past the seal without damaging it?


Luke Skywalker
Hi Ian,

First, go to a local auto parts store and get some Raybestoes "Brake Assembly Oil"... This stuff is much better to work with than brake fluid! And, it's compatible with all types of brake fluids.

Next, yes it's tricky initially to get those dust seals in place. You can search the archives here for more details, I know it's been discussed, but in a nutshell:

Close the bleeder screw and block off the opposing piston hole, then use *gentle* compressed air blown into the pipe connection to pop or "baloon" the rubber seal outward around the face of the piston. Note that only a gentle amount of air is needed, not a full blast.

A dull-pointed tool made from copper wire or a wooden popsickle stick or some sort of hard plastic can be very helpful getting the seal around the piston, too. You do need to be careful not to damage the seal, as you noted. That's why I suggest a relatively dull homemade tool.

After one side is done, lightly clamp it so that compressed air won't pop it back out, while working on the opposite side.

What I use to block the open piston hole are a couple plates of 1/8"x3" steel that are cut to the right length to extend a little beyond the caliper and allow me to use some C-clamps on them, with a piece of sheet rubber under the steel at the piston hole to form a reasonably good (if temporary) seal.

Note: You are well beyond this point, but the same plate steel and C-clamps are handy during disassembly, too. These will keep one piston in place while starting the other out with compressed air. No rubber seal is needed in this case. I pop one piston partways out, while the other is clamped. Then reverse the process and clamp the partially removed piston while using air to dislodge the second piston.

Hope this helps!


Geo Hahn

Country flag
What I use to block the open piston hole are a couple plates of 1/8"x3" steel...

[/ QUOTE ]

But if you don't have a suitable piece of steel handy you can insert the opposite piston w/o it's boot to block the other side.
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