MECHANICAL OVERHAUL

of AC's post-war 2 Litre Saloon

Mechanical Rear Brakes


Introduction

Chassis prefixed L or EL have Girling Non-Servo (GNS) mechanical brakes at the rear. Although all-hydraulic brakes are often seen as a plus point, I think the mechanical rear brakes are better. They provide the equivalent of a split-circuit hydraulic system, since failure of either front or rear brakes does not disable the other system. I also found the mechanical brakes to be more reliable than my front hydraulic system (the earlier hydrastatic type). With AC's own compensator on the rear axle, they balance excellently. They also don't seem to wear out! The biggest drawback is obtaining spares, not that you are likely to need many. The other drawback is a lack of understanding of these brakes, leading to incorrect assembly or adjustment. There is little information available on this type of brake, and the AC handbook is of limited help.

Description



The brake pedal operates a balance linkage between the master-cylinder and the brake-rod. The rod turns a cross-shaft in the centre of the chassis, which in turn pulls a cable. The cable operates the AC compensator mounted on the rear axle, which then pulls two rods, one to each brake. Each of these pulls an expander unit which resembles an hydraulic cylinder, and has two plungers that push out the shoes. Note that the expander is mounted onto the backplate in slotted holes allowing it to 'float'. Its mounting nuts (self-locking) should not be fully tighted. The reason for this floating mount, is to equalise the forces on each shoe, which increases the force on the leading shoe. This is basically the same principle as hydraulic cylinders which also provide equal forces. Under the nuts are double-coil "thackray" washers that maintain the required loading. I obtained thin self-locking 5/16BSF nuts from LAS Aero. At the opposite end of the shoes is the adjuster for taking up lining wear.



Servicing

The only servicing listed in the handbook is adjusting for lining wear. There is a square rod on the backplate which you turn with a spanner (while the brakes are off) until the linings touch the drums and the wheel won't turn. Then back off the adjuster 2 clicks or until the wheels spin freely.







Overhauling

The most likely problem after many years of use, is that the rubber gaiters have perished, and dirt may have contaminated the grease. The grease itself might be life expired. If the expanders are not able to slide on the backplates, the brake force will be substantially reduced. Disconnect the brake-rod by the shorter barrel nuts behind the backplate (don't disturb the longer adjusting nuts unless you have to). Remove the shoes by prising them off and being careful not to injure yourself, since the springs are very strong. I used a strip of plywood as a lever, plus a large screwdriver. Remove the expanders and the adjusters from the backplates. Remove the split-pins from the expanders, dismantle them (be careful not to drop the little rollers that reduce the friction between cone and plungers). Clean the parts and re-grease with mechanical brake grease (colour-coded green) and reassemble with new split-pins. The adjusters should simply come apart, unless the screw thread has seized. Clean up and regrease these too.



Refit the expanders with some brake grease behind them to allow them to slide on the backplates. Fit light-gauge washers over the mounting studs, followed by double-coil "thackray" washers and thin self-locking nuts. Don't fully tighten them, so that the washers are not fully compressed. Tightening until the nuts are flush with the ends of the studs, appears to be just right. I know it's standard safety practice for studs/bolts to poke out from nuts, but this is a special case. The light-gauge washers mentioned also help prevent the mountings locking up (because thicker washers will cause the double-spring washers to compress more). At the time of writing (October 2014) new rubber gaiters are available from Paul Beck Vintage Supplies.



Compensator



The compensator is largely packed with ordinary chassis grease. Dismantling is straight-forward. Clean up every part, smother the parts with fresh grease, and reassemble with new split-pins. While it is in pieces, you can clean and regrease the brake cable, without having to dismantle it (since you can slide it through the conduit far enough to clean). The compensator casing is held together with 3 unusual flanged studs, 2 of which double as pivots for the bellcranks (I think the flanges are soldered on). On my unit, they were fitted with self-locking nuts and spring washers. Spring washers should not be fitted under such nuts. Confusingly, while these studs are 3/8 BSF threaded, the small bolts that also fasten the casing, are 2BA items (also with self-locking nuts).

I have not been able to find suitable rubber boots for the compensator, and so I obtained large boots for trackrod ends and slid rubber fuel hose over the brake-rods (because the boots had holes too large). Small hose-clips prevent the fuel hose pieces from slipping on the brake-rods.



If you need to dismantle the brake-rods, then when you unscrew the long adjusting barrel nuts, count how many turns it takes, and make notes. Also remember that one end of each adjusting nut (and its locking nut) has a left-handed thread. I thoroughly cleaned up and painted the rods and nuts (excluding the threads) and renewed the RH threaded lock-nuts with new zinc-plated items.

The front end of the brake cable, has a clevis screwed on, and you need to note how many turns it takes to unscrew, to avoid loosing the correct adjustment. The clevises installed in the compensator (at the end of each brake rod), should have the rod screwed in until its tip is flush with the inner surface of the clevis.



Reassembly

I found that the set-screws securing the adjusters to the backplates, were a non-standard length, 15/16 inch. 1 inch screws bottom in the blind screw holes (with the spring washers included), so I added a plain washer to each new 1 inch long set-screw I fitted.

For cleaning up components exposed to brake dust, use brake cleaning fluid.

The final task is fitting the shoes. Mine looked hardly bedded in - let alone worn - even though they had done at least several thousand miles. That might mean that they were not providing the full braking forces for a long time. Refitting shoes is always a struggle. The springs have to be hooked onto both shoes before installing them. My technique was to then hook the top shoe in place, and then prise the lower one onto the adjuster with a large screwdriver. Then I used a length of plywood to press the lower shoe down, and with the help of the screwdriver, slip it onto the expander. Be careful not to injure your fingers if the shoes slip off.

After fitting shoes, you might find that the drums are a tight fit even with the adjuster slackened right off. You need to centre the adjuster by slackening its mounting set-screws. Then fully tighten the adjuster to lock up the brakes, then retighten the set-screws, and slacken the adjuster. Adjust the steady-posts by trial and error. With the adjuster, get the shoes touching lightly, then you can adjust the steady-posts until there is little or no drag. Then finally set the adjuster by tightening until the linings start to drag, and then back the adjuster off by two clicks.

My rear drums were secured with countersunk screws with a Whitworth thread, even though the front drum screws have a BSF thread.

The longer of the brake rods across the axle, tends to rub against the back of the differential casing. There is a fair sized gap, but evidently the rod flexes enough to strike the axle, judging by the marks it leaves. I added a slitted piece of fuel hose over the brake rod to prevent wear taking place.



In the above photo, you can see that the brake-rod touches (or nearly touches) one of the bolts securing the axle hasp, when the axle bottoms. It's recommended that a washer is added under the head of the offending bolt to provide some clearance.



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Website started 29th December 2006