The rear brake pressure limiter is a small but vital part of the brake system on our coupes. Yet BMW barely speaks of it at all in their original workshop manual, by basically stating “Do not touch”. That’s great, until it no longer works. Thankfully, this technical article will explain to you how it works, and if it doesn’t, how to disassemble, clean, repair, and reassemble the unit.

The limiter was first introduced when BMW changed to rear disc brakes on the 3-litre models in April 1971 to stop the car from completely locking up at the rear under (heavy) braking. Unfortunately, despite its importance, the system is very complicated if not almost impossible to check without sophisticated pressure-testing equipment. It is worth trying to find a specialist repair facility if you think your system might have failed. However, if your system is still working and simply requires a good clean and overhaul, keep reading on.

OPERATION
Before we start taking things apart, let’s take a quick look at how the system actually operates. (1) The limiter works by having a single piston which has two different-sized halves to it, also known as a stepped piston. Full brake pressure is applied to the larger face of the piston which pushes into a strong adjustable spring located inside the housing. The spring absorbs an increasing amount of the pressure as you apply more brake pressure with your foot. Meanwhile, the remaining pressure is transferred to the rear brakes by the smaller part of the piston. The piston is also equipped with a non-return valve, which allows brake fluid to pass through the limiter as the brake pads wear down over time. It’s a nice feature, although it does make bleeding the rear brakes a bit difficult if you’re running the system at too high a pressure.

So, now that the introductory part is over, it’s time to get our hands dirty! But before we do, make sure you’ve got a pair of disposable gloves at the ready, as brake fluid is nasty stuff! The BMW workshop manual (2) is very vague on this component and covers the entire subject in less than five sentences. I’ll try to do it in a few more.

A quick note: Some of you may have already seen my video which covers this subject. In this, I made a few mistakes along the way and had to redo some steps. I’ve mentioned these in the text that follows to make the process a bit easier for you.

REMOVAL
The regulator housing is located on the left-hand side of the car, just above the rear-diff. mounting. (3) It’s not very easy to access, especially if the diff. is still in place and you’re restricted in raising the car. However, even with the use of a jack you should be able to create enough room to get underneath the car. Just make sure you have the necessary safety precautions in place!

The first step is to drain the brake fluid as best as you can, especially if the rear diff. is still fitted to your car – this will prevent a lot of cleaning up later. Once the system is drained as best as possible, you’ll have to remove the brake lines. A special brake-line wrench (size 14) (4) is highly recommended to avoid damaging the brake lines and their fittings. Residual fluid will leak out, so make sure you have plenty of blue roll or kitchen paper nearby. Once the brake lines have been disconnected, you can move on to step two, removing the housing itself.

The housing is mounted to the car by two M6 bolts (at the top and bottom of the mounting bracket) (3), which require a size 13 socket to remove. Possible use of WD40 might be required if they’re rusty or difficult to undo. You should now be able to remove the system from your car.

DISASSEMBLY
The entire regulator is held together by four M5 bolts. The original workshop manual image (3) shows them as ‘Alan’ type bolts, however mine was fitted with normal hexagon-style bolts. It doesn’t really matter which of the two have been used, as long as you’ve checked their condition before reinstalling them later. Be mindful when undoing these bolts, as the internal spring will cause the housing and mounting bracket to “eject”, resulting in all the internal bits spreading out over your work space. Replacing two opposite bolts with longer ones of 60mm and screwing them tight will prevent this from happening as you can now slowly “unwind” them to open the housing. There will undoubtedly be some brake fluid left inside as well, so remember to place some extra paper towels on your bench. Once the last bolt has been removed, you can remove the mounting bracket and open up the housing which will reveal the internal components – the piston itself (which we’ll remove shortly), the spring, and two ‘caps’; one located at either end of the spring (5).

We’ll place these parts to the side and move our attention to the other half of the housing, the regulator body, better known as the bit where the brake lines attach (6). Located at the end is a large nut (wrench size 32) which we need to remove in order to extract the piston. Be careful not to lose the rubber O-ring that’s sandwiched in-between the nut and the housing. Once the nut is removed, you can extract the piston itself – don’t lose the second (smaller) O-ring located on the piston!

Finally, you’ll notice an adjustment screw/nut at the end of the spring housing (7). Whatever you do, do not adjust this! Of the five sentences BMW writes on this subject, this is the one that’s highlighted in bold – “Never attempt to alter the setting of the brake pressure limiter”. Pretty clear to me, I think.

CLEANING & REPAINTING
By now, we should have all the individual parts laying on our work space (8), and depending on the state of them, we’re left with two options. Option one is to continue cleaning the components, option two would be to go down the route of a full re-paint. If the parts are still in good nick and you’ve chosen option 1, a paper towel and a can of industrial brake cleaner will do the job nicely.  If, however, you’re in the unfortunate position like myself where more substantial rust and damage needs to be sorted first, we’ll need to look for a blasting- and coating facility next. You might find there’s a DIY-type workshop nearby – basically these guys allow you to use all their tools and machines for a monthly membership fee. It’s a really good way of getting access to expensive equipment and tooling on a small budget.

It is vital that before using any type of blaster, you’ve covered the brake-line openings (a simple bolt or cap will do the job) and you’ve put the large nut back in place to prevent damaging the internals. Once the housing parts are all clean, it’s ready to paint. One note of warning, be absolutely sure you’ve taped up all the parts that don’t need paint (i.e., internals) with aluminium tape, as it prevents paint from sticking to it. I opted for powder coating, but conventional paint is fine, too. Since BMW doesn’t allow us to alter the adjustment screw anyway, I simply left it in place and painted over it. This will hopefully also function as a clear indicator when it’s undoing itself by showing bare metal (or so I hope).

ASSEMBLY
After all the labour-intensive steps above, we’ve finally arrived at the point where we can reassemble the system. Now is the time to check the condition of the bolts and O-rings one last time and replace them if needed (ensure the latter is suitable for brake fluids). I’ve added the breakdown image (9) here as well, as it clearly shows all the components and their location withing the regulator. We’ll work our way from left to right.

First, we’ll have to undo the large nut (I re-installed mine before painting to stop paint from entering the housing) in order to re-install the piston and large O-ring. A quick wipe with a paper towel is useful to remove any dust or dirt that may have entered while everything was opened up. Once it’s all clean, we can insert the piston with the thinner side first – be sure to push it in all the way – before screwing the nut back on, not forgetting the O-ring like I did (up to 3 times!). Don’t overdo tightening the nut, as it may damage the O-ring and result in a leak later on. This completes the assembly of the main body, which we can put to the side for now. (10)

You’ll most likely find that your housing requires a new gasket between the main body we’ve just assembled and the spring housing. Mine was damaged badly and was removed completely during the blasting process. Unfortunately, I couldn’t find any online, so I had to make my own. It’s a simple process and requires generic gasket paper, a pencil, scissors, and possibly a knife and 6mm drill. Using the spring housing, a template can be drawn which can then be cut using the scissors. (11)

The centre hole is slightly more difficult – I did this by laying the paper on top of the housing and rubbing a pencil against the “inside”. While applying pressure, a (vague) line should appear, which we can then trace to get the correct template. Hopefully the images (12) and (13) will show what I mean.

You might find that the PBM Performance kit that is meant for the Porsche 914 also fits the E9 regulator, which would save you the work of having to make your own gasket, and also comes with new O-rings and bolts.

With the new gasket made up, we can start the assembly of the spring system. I found that standing the main body on the workbench worked best, supporting it with my left hand, while placing each subsequent component in order with my right. First, the large cap is installed over the piston. Next, you can slide the spring on top of this cap. Lastly, the smaller cap is placed on top the other end of the spring. Don’t forget to install your freshly made gasket before sliding the housing over the spring! (14)

Before bolting the lot together, refer to the initial image at the start of this article where it was still mounted to the car to ensure you’ve positioned it correctly. Because of the spring tension and the short length of the retaining bolts, it can be pretty difficult to get everything lined up. I used two longer bolts as guides while pushing down on the housing. Using a socket wrench 13, the shorter bolts can be screwed in, which should hold everything together.

As an extra safety precaution, I used Loctite (638) to stop the bolts from undoing themselves when vibrating while the car is in motion. I might be overcautious, but I think it’s worth doing while we have easy access to them. There’s no mention of any torque figures for these bolts, so to quote a famous engineer (my Dad) “Tight is tight”. I actually went as far as applying torque seal (15) on all the bolts – something I’m doing throughout the car, especially on the engine block.

This stuff is a bright-red rubber-type paste that hardens and will break when a bolt unscrews itself even a small amount, allowing for easy spotting of loose bolts. We use it intensively in aviation, where my background is from, and if you’re planning on driving the car more than just a summer’s weekend, it’s worth considering.

MOUNTING AND A FINAL WORD OF CAUTION
Unfortunately, there’s no way of testing the system for any leaks until we’ve reinstalled it on the car and connected all the brake lines. Aside from keeping our fingers crossed we’ve done a good job, there’s nothing else we can do at this point.

Which brings us to the last step in the overhauling process, bolting the regulator system back onto the car. This should be pretty straightforward and is a reversal of the removal process we did at the start of the article. Just remember: Before taking the car out for a spin to show off your freshly overhauled “rear brake pressure regulator” to all your friends, make sure to bleed the brake system first.

Gute Fahrt!