Brakes

[el torro]

Is there a tried and tested way to bleed brakes,do you need engine running to get servo working etc

[Esprit2]

The power brake servo is external to the hydraulic system, and has nothing to do with bleeding the brakes. It reduces the effort required to depress the pedal, but it has no direct effect on either the fluid or air in the system. It's not necessary to have the engine running while bleeding the brakes, but it doesn't cause a problem.

In normal use, the master cylinder's piston & seal only use a portion of the available stroke. Over time, the bore wall can wear in that short-stroke area to the point that a step develops. But in normal use, the seal runs within the worn area and isn't forced to cross the step, so no harm, no fowl. However, the full stroke is used when bleeding the brakes by pumping the pedal. In that case, forcing the seal back and forth over the step can damage the seal to the point of failure. For that reason, I don't pump the pedal to bleed brakes.

There are two separate subjects at work here. First is how to bleed the brakes, and secondarily, master cylinder maintenance. I'm not saying it's okay to let the brakes go long enough that a large, damaging step develops. The brakes are a safety system, and you should stay ahead of the required maintenance. That includes rebuilding the master cylinder as required. However, I am acknowledging that steps do develop as a part of normal wear, and suggesting that not pumping the pedal to bleed the brakes simply avoids an obvious problem. If you know a brick wall is there, no rule says you have to butt your head against it.

My favorite method for bleeding the brakes is to use a pressure bleeder, like a Gunson Ezi-Bleed. However, keep the bleed pressure low, in the 5-10 psi range. The Gunson instructions suggest using the spare tire as a pressure source, but neglect to recommend reducing the inflation pressure to 5-10 psi first. If you use full normal tire pressure, it's very likely that the cap will blow off the reservoir, and spray brake fluid all over the place. Not only is that messy, but brake fluid loves to eat automotive paint. Treat brake fluid like a hazardous material around paint.

I use a cheap, 2 quart garden sprayer as a pressure source. A hose goes from the bottle to a spraying wand. I simply removed the wand, and connected the hose to the reservoir pressure cap provided in the Ezi-Bleed kit. Pump up the garden sprayer enough to produce a rate of flow fast enough to carry air bubbles along with it, but don't use excessive pressure.

In the past, I've also used a vacuum bleeder, such as the Mity-Vac. They're very effective at pulling brake fluid through the system fast enough to carry all the air bubbles along with the flow. The problem is that when the bleed valve is opened, air can (usually does) flow in past the threads, where it does a quick U-turn and comes right back out the bleed valve. As a result, you can vacuum bleed the brakes until the cows come home, and you'll continue to see a flow of small, fine air bubbles in the clear bleeder hose. The trick lies in learning to differentiate between the really fine 'air leak' bubbles and the larger 'entrained air' bubbles. Stop when the big bubbles disappear, and try the pedal to see if it's firm.

Wrapping the bleed valve's threads with Teflon tape can significantly reduce the amount of air that's drawn in past the bleed valve. Don't use a liquid thread sealant, since the sealant itself can be drawn into the system, contaminating the brake fluid.

This next bit is a little off-topic, but the Europa's master cylinder is mounted very low, just off the floor of the front boot. That puts the rest of the plumbing high above the reservoir, and high points are air bubble traps. Bleeding the Europa's brakes can be a real excersize in frustration. For that, I use the pressure bleeder on the reservoir, plus a Mity-Vac vacuum bleeder at the caliper's or wheel cylinder's bleed valve. The push me/ pull you approach works first time every time.

Regardless of the method you use, start with the wheel that is farthest from the master cylinder, and progress to the closest wheel last.

For systems equipped with a Pressure Differential Warning Valve (PDWV), the bleeding process will cause it's internal shuttle to move off-center, close the electrical contacts, and illuminate the Brake Warning Light on the instrument panel. When you're done bleeding the brakes and satisfied that the pedal is firm, go back and bleed one wheel one more time in order to center the PDWV. If you finished bleeding at one of the front wheels, then the PDWV's shuttle is displaced toward the rear circuit. So go to one of the rear wheels and bleed it just a little (no matter which end of the car you're at when you finish bleeding, go the opposite end to center the PDWV). With the ignition switched "On" so that the warning light is illuminated, crack the bleed valve open just a little, and slowly bleed off some fluid while keeping an eye on the warning light (a helper can be handy). Stop as soon as the warning light goes out, and tighten the bleed valve.

Regards,
Tim Engel
Lotus Owners Oftha North (LOON)

[stugilmour]

Excellent write up Tim. Thanks for listing.

Stu

[mjbeanie]

Thank you Tim,

Excellent summary and technical overview. I really needed the info on the PDWA since I finished the bleed and now have the infamous brake fail light. Thanks again

[Andy8421]

The power brake servo is external to the hydraulic system, and has nothing to do with bleeding the brakes. It reduces the effort required to depress the pedal, but it has no direct effect on either the fluid or air in the system.

Tim, on many modern cars this is true, but not the case for the Elan where the servo is in the hydraulic circuit and is effectively a pressure amplifier.

On the Elan, the hydraulic fluid passes through the servo on its way to the calipers, and if air (say) was trapped in the servo then this would impact the overall system performance. The servo gets bled along with the calipers as part of the normal bleeding process.

Andy.