My transmission has developed a pretty serious leak that seems to be coming from this allen bolt in the picture. I don't have the proper sized allen bolt to try torquing it down with, does anyone know the size? Is there a seal or part behind that bolt that is simple to replace? Thanks

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'81 300SD

What side of the Transmission is it on?

There is 2 places on mine where you can hook up a Gauge maybe that is one of the Plugs for that.

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84 300D, 82 Volvo 244Gl Diesel

It's on the driver's side of the transmission. I cleaned up the area really well with brake cleaner and a paper towel, took the car around the block and found oil coming out of the huge allen bolt and a little bit on the gasket about 2 inches towards the rear. That allen bolt won't budge. I'm starting to think that the trans-x I put in about 8 months ago has thinned out the fluid to the point where it is leaking past the allen bolt threads. I'll do a fluid and filter change in the next week or so and see if that helps.

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'81 300SD

I'm pretty sure that part your looking at is the plug for the B1 Brake Band "reaction valve", there is no seal of any sort immediate to that plug, no O-ring or washer seal on the plug itself but there are a couple seals deeper in around the reaction valve itself which might be the source of the fluid leaking from the plug.

Id be careful with that plug because the Allen Head design is something that is notorious for stripping out when it's in aluminum.

I called an MB transmission shop about repairing that B1 brake band, and he recommended doing a full rebuild because the transmission has over 250K on it. I'm hoping to find an indie who can do the job for $300 or less.

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'81 300SD

I believe the basic rebuild kit is over $300. I would be suspicious of anything too cheap.

There is a thread here that does an excellent job of explaining the disassembly (so far). It would be a great reference to see what it would take to do the leak repair.

I hope nothing I said was seen to imply there is a problem with the B1 Brake Band itself or the reaction valve itself. I have know way of knowing what's going on in that tranny. If you go to the Mercedes EPC you can sort of make out the order or position of the relative parts. The plug appears to secure the reaction valve in place, there are a couple seals around the valve and nothing to seal anything at the plug. Stands to reason the inner seals under normal conditions do a sufficient job of preventing fluid leaking that the MB engineers didn't consider sealing the plug itself.

I'd suspect one or more of the inner seals has failed or some failure in the body of the reaction valve could be allowing fluid to leak to the unsealed plug. Either way I don't yet see how it could be the B1 Brake Band itself as the problem, unless of course it could be in some strange way were the band or one of its ends wears and breaks and then jams into place or something like that, but I'd expect to remove that plug and see the end of the reaction valve, one of the ASTG transmission manuals probably shows and tells how to get the valve out/disconnected it from the Band.

I mean the MB engineers used the plug probably because they anticipated either needing the opening for assembly or for adjustment of some kind or repair.

$300 is my ceiling for the B1 repair. There's a reputable shop out here that will rebuild and install the transmission for $1950.

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'81 300SD

I have a couple of mechanics in mind who I'm hoping can replace those seals for me.

One thing worth mentioning is that when I put the car into Drive or Reverse from a stop, it takes over one second for the transmission to engage. The last transmission that I had in this car would engage immediately. Could that hesitation in engagement have something to do with a worn out B1 band?

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'81 300SD

Any tips on busting this 14mm hex bolt for the B1 piston loose? I've tried a 16" breaker bar with an 8" pipe on it and no luck. I was thinking of getting the transmission hot, but someone mentioned earlier that the bolt is aluminum.

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'81 300SD

Is this car a daily driver? If not then there is a bit less concern. If it is I would step back a bit and try to do some research about exactly what getting into there might entail, you don't what to just start yanking things apart and potentially making a bigger problem for yourself. Here's something that I quickly found that might be a start (looks like there is another informative link inside that one as well)

R&R B1 Reaction Valve Cover - Mercedes-Benz Forum

In any case I would try and find a replacement plug before I attempted too much on the present one, are they available and what's the cost? That way you can better decide what is the most cost effective choice if stuff starts getting squirrely. That plug corrosion-bonded to tranny case would not be outside the realm of possibilities!

It's aluminum to aluminum so it can be a royal pain possibly. I would first give the end of the plug a couple good wraps from a hammer, you want to hit it straight square and as solid as you can, short fast hit to help shock the metal to metal interface. Then I would get someone to apply torque to the Allen Head and while they maintained good pressure use a large chisel and a 3 pound drilling hammer to move the plug counter clockwise. The combination of constant torque and hard whacks should break most things free.

Heat could be applied to the plug itself, nothing like red hot or anything like that, too hot to keep your finger on it might be plenty to get things loosened up. I'd want a small hot flame applied in a circular motion around the circumference of the plug. Try that once and if it didn't work I'd try it again a couple times letting it get cool in between heat applications, maybe heat it and then cool it fast with water, ice or air. What you want to accomplish is for the problem connection to cycle through the expansion and contraction process that will hopefully allow things to free up.

The one thing you probably won't want to do (except as a last resort) is just try to torque it out with only leverage, aluminum just doesn't have the strength to no deform at the tool interface.

One sort of trick when working with any fastener you're trying to remove that might be a bear is to first try to tighten it a tiny fraction. What that does is allow you to gain some movement using the tool-fastener surfaces that you won't be depending on to get the thing taken completely apart. Penetrants like KROIL or PB Blaster can often help but with aluminum parts the problem is often aluminum oxides that are the corrosion, and they are very hard stable compounds that don't change their structure easily, that's why they are used as abrasives.

Thanks for the tip. This car is a daily driver, and is the only vehicle I own. I'd like to try some high temp thread sealant on that bolt since the transmission works well, and the B1 band doesn't need to be messed with as far as I can tell, other than the seals. I have an extra transmission with a bad front pump on it in my garage. I tried to remove the 14mm hex bolt from that with no luck. I'll try some of the heat and hammer techniques that you mentioned on the spare transmission first.

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'81 300SD

I was able to remove the bolt from the spare transmission using one of those impact tools that you hold in your hand while whacking the socket into the bolt. I won't have the clearance under the car to use that tool, but maybe just tapping on the socket with a hammer will do the trick seeing how there is ATF all over those threads.

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'81 300SD

Could the leak be coming from the vacuum modulator or the oil cooler line banjo? I have never seen it leak on that big allen bolt.

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'85 300D Cal 280,000 miles
'14 GLK 350 60000 miles

like is mentioned in Pink Floyd's - US and Them "if you give 'em a quick sh...short, sharp shock," works!

I've always had and used the hand impact tool, usually works on difficult stuff like door hinge or latch's bolts and screws. Nice to be able to use the 5/16 driver bits with the 3/8" adapter to move big screws using a ratchet to drive them. But I would have been worried about punching through the back of the plug not knowing how much material was there, but obviously it survived what you've done. Plus 14 mm, that is pretty big to easily strip out.

Now you've got a plug out on your spare tranny, you could get a look at the reaction valve and whatever seals that might be the root of the problem leak.