Hello,
I am in need of rear calipers for my 1993 940 T. I am having trouble finding the rears, especially the left rear caliper. One question I am being asked is about caliper piston size. I tried to measure it but without taking the caliper apart, I am unable to get an accurate measurement. So, even after providing the VIN, I am not having any luck with this search with the local auto parts. The only numbers that I can find on the caliper is G 5 on one side, and 12 G on the other side of the ATE calipers.

Does it matter? If I do have the 38mm caliper, can I just use the 40mm caliper, or vice versa? Any help is appreciated. Thanks!

Pittroj

Here's a parts house I’ve had good luck with in the past. Looks like they have reman’s from several different rebuilders including Raybestos if name recognition matters. Looks like straight axle and multi-link use different calipers.

https://www.partsgeek.com/catalog/1993/volvo/940/brake/brake_caliper.html

Thanks everyone for all the great info. I took pics of the calipers before I put the wheels on and the round part with the stamp on it looks like I have the 40mm pistons. It is really rusty but if I zoom in, I think I am seeing a "4" and possibly a "0". I guess before I buy the rebuild kit, I will pull a wheel off again and use a file and get a better look. I am going to attempt to rebuild them on the car, any input on that? There are youtube videos of guys rebuilding them on the car but the opposing pistons may make it hard.

These calipers are getting scarce, and FCP has only the left rear( reman shown as having the 40mm piston). EEURO has no rear calipers for the 940T. I called TASCA and they ccouldn't tell me what size my rear calipers are even with the VIN. Thanks!

Yes, you can try doing a caliper rebuild on the car without disconnecting the brake hose if you really want, but I much prefer doing it properly on the work bench or even the garage floor where you can see what you're doing. On the car, things will be a lot fiddlier with the caliper dangling on the rotor and you very much risk damaging the brake hose if you accidentally drop the caliper. With a little penetrating oil you shouldn't have too much trouble cracking open the brake hose and removing it (the bleeder nipples are generally trickier and require more patience and finesse). Then you can remove it to the bench for proper cleaning, inspection and replacing the seals and dust covers. Either way, you need to minimize brake fluid loss to make re-bleeding the lines easier. Either use a hose clamp or protectively wrap the hose with rags and clamp off with locking pliers or plug the open hose end with a fine tapered plug, a fine screw, even a cap wrapped in tape will help slow the flow for an hour or two. A bicycle pump (or whatever) with a tapered beach ball adapter is often enough to blow the pistons out one side at a time, or you can blow them out on the car using pedal pressure, but it's a whole lot messier. Make sure the piston is kept dead square when pressing (clamping) the piston back in under the new seal otherwise you risk tearing/folding the seal -use lots of brake fluid as lube and clean up any excess later with spray brake cleaner. Pressure/vacuum bleeding recommended, but two person pedal bleeding is okay as long as you stay within the normal operating range of brake pedal travel to help prevent the seals from getting damaged in grungy untravelled areas. Unless your pistons are scored, this is generally every bit as good or better than a re-man caliper plus you know where they've been.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now

Another detail about the rear caliper pistons is that they may have a small “step-down” section on the surface that presses against the pad. Technically the piston is supposed to be clocked to a certain position that puts the step at a certain angle relative to the rotor path. This section from the 700/900 FAQ discusses it:

https://www.brickboard.com/FAQ/700-900/Brakes.htm#RearBrakePistonRotation

Hello Chris. There was a thread with Art Benstein earlier this year on stepped Ate rear pistons. As I recall, we more or less concluded that most if not all years of 240s with Ate rears had stepped pistons and most if not all 940s with Ate rears did not have the stepped pistons. As for 740s, I'm not sure we reached a conclusion, but I'm quite confident my '89 740 (the year ABS was introduced) did not have stepped pistons.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now

Interesting....

We had a 1990 740 without ABS and as I recall it DID have stepped pistons. Of course once a car reaches 10 or 15 years old there’s no telling what sort of replacement calipers may have been installed along the way. Especially given how inaccurate some of the auto parts houses parts catalogues are.

Hello Chris. Yes, interesting. It wouldn't at all surpise me if the change away from stepped pistons on Ate rear brakes coincided with the introducton of ABS brakes. The entire brake system had to be re-designed and re-balanced from a dual triangular semi-redundant brake circuit system (that Volvo helped pioneer) in order to support the dual front and single rear brake circuits used with ABS brakes -the fronts can sense and pulsate left/right while the rears can only sense together off the drive shaft and pulsate together. That meant new caliper designs all around, most notably the front calipers moving from a four piston design to the more modern single piston floating clam shell design. While this may have been true of the 740 series, I have my doubts about the 240 series as I've yet to see anyone mention non-stepped Ate rears in the later 240s, some of those later years supposedly having ABS. I'd be interested in hearing thoughts from others on that theory (Art?).
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now

For my part, I know zilch about what came on 7/9 cars.
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Art Benstein near Baltimore

“He who is not contented with what he has, would not be contented with what he would like to have.” -Socrates

Yes, you need to determine whether you have 38 or 40 mm pistons (or 36, which are less common). No matter what, the calipers need to match left/right, so unless you're replacing both calipers you need to be extra careful. I've always been of the general opinion that the later 940s (as in 1993-on) got 38mm in the sedans and 40mm in the wagons. Only a few months ago I was buying a caliper seal repair kit and needed to know the piston sizes. Eveyone kept talking about a 38 or 40 stamp, but I couldn't find it. I was just about ready to pull the caliper, but thanks to advice elsewhere I figured out where to look. Unless your caliper is horridly rusty, you should be able to spot the mark or at least a hint of one digit so that you can identify which of the three numbers it is.

First off, look for the raised "Ate" on the outer casting (ignore the raised G). Right next to that is raised cirular glat area about the size of a nickel. The 38 or 40 is stamped there. It's a very fine engraved stamp and not overly deep. My calipers were certainly rusty, but not heavily encrusted, and with a bit of effort I was able to spot the number.

Start by wire brushing the area clean (by hand or drill). Wet the surface with water or spit and see if you can now see it. Use a bright light, like a good LED flashlight. Normally you won't be able to see it yet, so take a flat file to the area (not a grinder). Keep wetting and checking as you go until you're down to shiny bare metal at which point you'll have to give up. It took me about three goes with a file and there it was plain as day after I wetted it.

Here's a really good pic from another site that helped me

Zoom in and stare hard at the circular area on the lower left and you'll see a fine 38. That's what you're looking for.

When all else fails, you will have to remove the caliper, pop off a dust cover and extend the piston a bit so you can check it properly with a measuring gauge. The difference between 38 and 40 isn't much, so trying to eyeball it with a straight edge ruler isn't good enough.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now

Again, thanks everyone for the help. I checked the round casting on the inside(nice of them to put it on the INSIDE, NOT)and, I do have the 40mm pistons/calipers. I am going to order a 40mm rebuild kit for the calipers. While I had the wheel off, I made sure that the bleeder screw will break loose, which it did. If it was stuck, I may have ordered calipers rather than rebuild kits. Thanks guys!

order from FCP they have long experience w volvos