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TR4/4A 66 tr4a front caliper rebuild...tips?

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lynfield66TR4A

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Hi everybody...
I am in the process of putting new pistons in new calipers.
Is there a special kind of lube that helps the piston over the seal?
One piston just kind of slid in, and I had to fight the other, and wouldn't you guess, it leaked. After marinating the garage in dot 3, in the process of removing offending piston with compressed air, holding in the good side, air pressure built up and fired the piston into the fingers holding the other piston, after the cloud of profanities cleared, i wondered if there is any easy way to install the piston without damaging the seal? Is there anyplace just to get the seals without buying the kit? Has anybody just used o rings? I remember using plain o rings in my Ducati clutch slave and it worked better than the oem seal and lots easier to install piston.
Thanks for the help,
Mike
 
Hi Mike,

The rule of thumb is if you have to force the piston in place, the seal has slipped out of position, will get pinched and will leak.

No, I don't think I'd use o-rings. The pressure can get pretty high in the brake hydraulics and I'd want the wide seal, rather than the narrow contact area of an o-ring. Also, what are the assurances an o-ring is made of material that's fully compatible with brake fluids? It's a pretty sure bet that the materials of a brake seal are compatible.

Using an o-ring for a clutch slave cylinder isn't too risky, IMHO. Look at it this way: If the clutch fails due to the o-ring, it just means you can't *go*. If an o-ring is used in the brakes and they fail because of it, you can't *stop*. Which is the worse scenario?

Sorry, I don't know of any seals available separately. On the other hand, a rebuild kit isn't expensive and will provide spares for any future servicing.

There is "brake assembly lube" which helps a lot and is much better to work with than brake fluid. Raybestoes makes it, maybe others, too. It's essentially a thin oil that's compatible with all types of brake fluid.

BTW, I've had many a bruised knuckle and uttered my share of profanities working on calipers, too.

/ubbthreads/images/graemlins/cheers.gif
 
Square cut "O-Rings" are used in 99.9% of the calipers out there. One of the reasons they are used is that as the piston is forced out of the bore, the seal sort of hangs onto the piston and distorts somewhat. When the pressure releases, this distortion wants to return to it's original shape and pulls the piston back with it. This is provides a bit of clearance for the pads so they don't drag too much. There is, of course a limit on how much distortion will take place before the piston slides past to compensate for brake wear.

As far as assembling the caliper, I would recheck the groove where the seal is. If there is any rust of foreign matter behind the seal, it will cause the seal to stick out at that point. The piston will then shear that section off as you poush it in. Look at the bad seal and see if there is a small chunk cut off. That's a dead give away the the seal groove has some crap in it. Also, there are brake assembly fluids available that are available. The fluid is VERY SLICK, and make putting the caliper together much easier. One more thing...installing the dust boot...again, the groove must be very clean. Install the boot on the PISTON first. slide it down so that it is past the bottom of the piston, but still on it. THEN wiggle the boot into the groove, and push it into the bore.It should be much easier to do this way then installing the boot in the caliper first.
 
I also use the brake assembly lube... sort of red snot that stays where you put it and helps it slip together.

The dust boot was always the tricky bit I thought. Never had any luck putting it on the piston first though. I put it in the cylinder groove then use some gentle air pressure to inflate the boot and pop it around the piston as the piston is held against it. Complex but effective.
 
use the lube, regulate the air pressure down low- 5-10 psi, once the boot is over the piston take the air off. make sure the piston is square. use a c-clamp with a piece of flat bar across the piston to GENTLY squeeze the piston in, keeping it square.if the boot needs correcting, put a block on the piston to keep it in place and add air pressure again.
rob
 
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