Friday, June 17, 2016

Leaking ABS modulators on 1990s Rolls-Royce and Bentley motorcars


By this juncture, wise owners of vintage Rolls-Royce and Bentley have learned to take fluid leaks in stride. It’s part of the charm, to keep the garage floor lubricated. Folks who could not handle the leaks bought Toyotas instead.

But we assume the leaks won’t strand or disable our cars. We particularly assume the leaks won’t cause us to crash. That is what makes this particular story so distressing.

The 1997 car shown here arrived at our shop with some common complaints. The brakes were soft, and pulled to one side and the low fluid warning was coming o. It seemed routine – add some fluid and bleed the nitrogen bubbles out of the system. We encourage people to do this service every year, in the spring, because nitrogen gas leaks into the fluid slowly as the accumulators age.

But that didn’t solve this car’s problem. Bleeding did fix the pull, but only temporarily. A closer look revealed the problem. The ABS modulator was leaking, allowing fluid to escape (hence the low fluid warning) and allowing air to get into one of the front brake circuits (hence the pull to one side.)

The fix for that was obvious – a new modulator. That was when I had a rude surprise. I went to the heritage.bentleymotors website and entered the VIN. Then I typed “ABS modulator” into the search box. The part I was seeking came right up – the UR27685. As is often the case, the website said, “dealer to advise on cost and availability.” Unfortunately, the dealer found no cost, and no availability. The part was discontinued.

I was rather surprised that Crewe would simply drop an essential part that’s used in most 1990s sedans. But that seems to be the case. A search online revealed it’s out of stock at the aftermarket sites too. I hated to do it, but we went to the used parts people and got a “pre-owned” brake modulator. It leaked too.



With that we realized that some new solution was needed. Fixing the leaking modulators is not an option, because they are assembled in a way that precludes disassembly. The other issue is that repair parts are not sold. That means anyone repairing the modulator would have to assume responsibility for testing and validating critical brake system components.

After considerable thought and document review, we settled on what we think is a better option – we changed the hydraulic plumbing to the configuration RR/B had immediately before the addition of ABS.

The original Rolls-Royce hydraulic brake system was extremely complex – un-necessarily so, as it turned out. They had two pumps pressurizing two reservoirs, and those reservoirs were pipes to two sets of distribution valves, lines, and brake calipers such that each wheel had two essentially independent brake systems stopping it.

By the late 1980s the engineers at Crewe had quietly backed away from that in favor of a system where one hydraulic circuit powered the front brakes, and the other circuit powered the rear brakes and the level control. The cars still had two brake calipers on the front wheels, but they no longer had two lines feeding them. Now they were fed with one circuit.

When you look at an older Rolls (like the Shadow in the top photo below) there are two completely independent circuits for each wheel.  By the 1990s (lower photo, Continental R) it looked the same superficially but the two calipers were now fed from one hose.




This was the hydraulic system RR added antilock functionality to in the late 1980s. Returning the ABS-equipped car to that basic design turns out to be fairly straightforward. The modulator has two “inputs,” from brake circuits 1 and 2. It has outputs for the left and right front, and the rear. It also has returns to vent excess pressurized fluid back to the reservoir when ABS activates.



We made up a series of pipe adapters to take the place of the modulator. The benefit of this is that we don’t have to engineer and validate something new. We’re returning the car to the previous iteration of the Crewe brake design. Given the choice of no ABS versus a system that leaks, becomes unsafe, and will eventually fail entirely, the choice is obvious.

And for most owners, the elimination of another $3,000 part that can fail is a welcome simplification.

As these cars age, it is certain that other parts will be discontinued and we will face similar situations. The best we can hope for is to make wise choices and hope for simple modifications with good outcomes. This appears to be such an example.

Thanks to vintage Rolls-Royce and Bentley master technician Bud Orlich for figuring this one out!

John Elder Robison

(c) 2016 John Elder Robison
John Elder Robison is the general manager of J E Robison Service Company, celebrating 30 years of independent Land Rover, Rolls-Royce and Bentley restoration and repair in Springfield, Massachusetts.  John is a longtime technical consultant to the Land Rover, Rolls-Royce and Bentley clubs, and he’s owned and restored many fine British motorcars.  Find him online at www.robisonservice.com or in the real world at 413-785-1665

Reading this article will make you smarter, especially when it comes to car stuff.  So it's good for you.  But don't take that too far - printing and eating it will probably make you sick.

1 comment:

Kendall Larsen said...

ABS is the crucial part of any car. If this type of issue done then how to overcome if it cause on long route. This is not good and done with this type of brand name cars.