Cars / e32 / Brakes / Breakdown of the H31 Brake Booster

Breakdown of the H31 Brake Booster

Article by: MaxF

Applies to: early E24, E31 (850), and early E32 (735 and 750) models.

While I had the chance, I thought I’d take the old ATE H31 booster apart to see if they are really as “non-rebuildable” as everyone says. . . 

I’m also aware of those of you who CAN’T “go vacuum” simply because the engine bay is too tight (eg E32 750s, and the E31 850s where the V12 makes things “cozy”), and so would be interested in any prospects of a rebuild process.

Here’s the exploded view:

Remember. too, that my car has done 160,000 miles, and I’m guessing this is probably the original booster…

Original reason for failure: leaking from the brake master cylinder end, eg a failure of the seal in the low-pressure end cap K to the output push rod G.

Quick thoughts on rebuilding:

a) if your booster was leaking from the pedal shaft, the high pressure seal probably isn’t renewable (Beware – some oil present in the felt ring under the rubber cap is normal – mine was “misted” or just oily)

b) check the body for scoring in the bore

c) check the white high pressure seal rings on the servo piston

If any of a, b or c are bad, I’m guessing they’re not common to find replacements for…

d) see if you can get a replacement seal for the end cap, K, and some oil-resistant adhesive to seal it in. I’ve not tried this, but K shouldn’t see much more than atmospheric pressure anyway

e) get the other 4 internal o-rings (J, 2 from D, 1 from E) and replace

How to get it apart:

To get your booster in this state, you’ve got to use vice-grips to loosen that little grub-screw on the bottom of the body – it holds the end cap K in against the pressure of return spring H.

BE CAREFUL – that’s a powerful spring, so use a rag over the end to stop bits flying and don’t put your face close!!

As far as rebuilding chances go, the bore in the body A and the pedal shaft coming from the servo piston B are all in great, un-scored condition.

I guess the special hard-teflon (I **think** it’s teflon) seals – the white rings on B, and another in the body – are OK, and helped this.

They aren’t standard, off-the-shelf parts, and if they were damaged I’d say you would have a hard time replacing them.

Here’s the bore of the body, for reference:

Not bad, eh?

The main culprit for rebuilding would be the end cap K – here’s some close-ups, the seal seems to be a normal kind of seal, but possibly glued into the plastic end cap itself:

Remember too that the seal for the outer edge of the end cap (to the bore) is via o-ring J which is compressed between the step on the end cap inside end and the spring-seat washer I.

All the other items do have varying seals, but all of the common o-ring types – since we’re using common ATF I don’t think they’re any special material, probably just normal neoprene.

The control piston D has 2 on it; it’s usually stuck inside the servo piston B, with the push rod seat E stuck into its end (E has one o-ring on that “stuck” end).

Wiggle the end of the piston, and spring C will push the lot out.

Here’s these parts in close-up:

And here’s where the fit into:

Still thinking on the rebuilding side, here’s the details of the high-pressure end of the servo piston:

I think this end is not dismantle-able, it seems to be “staked” (eg edges stamped over) together.

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