Hydratech Braking Systems - Brake Bleeding 101

Brake bleeding is frustrating, especially with all kinds of new components. The biggest problem is tiny champagne bubbles throughout the caliper castings, brake lines, junctions, fittings, etc… Rapping on the calipers firmly with a rubber mallet helps to jog loose the majority of the champagne bubbles trapped along the rough cast interior walls of the calipers. You are where you need to be though, as you indicate that the pedal gets firm at about the 2” point in the travel, which indicates that you do indeed have the preliminary bleed sequence accomplished. The hydroboost units are actually the opposite of a vacuum booster, in that when they are powered down = the pedal height during apply will be about an inch lower versus when the assist unit is powered up (engine running).

What to do next? I’d move on to other constructive aspects of your build, knowing that your brakes are 95 % bled. Once you are ready to actually have it move under its own power, I suggest you securely raise the rear wheels off of the ground, then apply engine power to the rear wheels. With the rear wheels spinning, then verify that you can conclusively overcome engine torque with the braking actions = proves that you can safely stop the vehicle during a maiden voyage. What you will then do is to run the vehicle carefully down the road a bit, then re-bleed the brakes further. Actually running the vehicle down the road is key to ridding the brake system of the remainder of the air trapped in various parts of the brake system. The vibrations and jounces produced by running the car down the road help to shake loose the last bit of air trapped throughout the system, along with the temperatures produced at the wheels in real time braking actions.

It is always amazing that right when you think you have the system 100 % bled, you will always be able to coax just that last little bit more of air out of the brakes after the first true road test. Please also keep in mind that all brand new brakes at the wheels will always be initially “soft” until the pads fully bed in to the rotors and all of the new machinery gets truly run in. This applies to most any car, rolling off the assembly line brand new or recently having the brakes relined / serviced. Typically the pads will wear / bed in within the first 100 miles, though may in some cases take up to 500 miles (depending upon the pad compounds and machined finish on the rotors). You will know the pads have fully bedded in when you can no longer see the machining marks on the friction areas of the rotors. Pedal feel also changes dramatically as the pads bed in, resulting in a crisper / firmer pedal response. Also note that the hydraulic brake assist unit will need to break in somewhat too, as we set these units up with the tightest possible clearances through a careful select fit process. You can count on some softer brake pedal response until the assist unit loosens up a tad, the air fully purges out of the PS system, the brakes are re-bled after initial road testing, and the pads bed in. Excellent results can be typically found at the 70-100 mile mark of actual road use!

Brake bleeding tips:

*Always thoroughly bench bleed a brake master cylinder before installation. The old school hose method is still acceptable, though the latest methods of brake bleeding with the ports plugged is actually superior in most cases. With the brake line ports plugged, every time you stroke the piston inward you will be compressing the air in the cylinders. Air raises in temperature when compressed, and when you release the MC piston it will push out of the compensation ports down in the bottom of the brake fluid wells. If your eyes are sharp, you will see these air bubbles through the brake fluid sitting in the compensation ports down at the bottom of the fluid wells (looking like silver BB’s in the drilled passages). You can sometimes coax these bubbles out very conclusively by only stroking the MC piston inward about a ¼” at a time in rapid succession. With the port plug method of MC bleeding, you cannot get a “false positive” like you can with the hose method, as you know beyond any shadow of any doubt that the MC is fully bled out when you can no longer stroke the MC piston inward more than about a ¼” as the fluid simply has nowhere to go and fluid will not compress. If you can still feel a sponginess to the piston apply by hand, then there is still air trapped in the MC. If it’s spongy in the vise during a bench bleed, it’s going to be spongy once installed on the car too…

* Gravity bleeding is possibly one of the simplest and most effective methods of brake bleeding. Note that this only applies to applications where the MC is considerably higher than the brakes at the wheels (firewall mounted systems). Unfortunately under floor mounted street rod type systems cannot enjoy the simplicity of a gravity bleed. With the thoroughly bench bled MC being the highest part of the system, you can simply open up the bleeder screws at the wheels and sit back and let gravity and capillary brake line actions do the work! Keep your eyes on the fluid levels in the MC at all times though, as if you were to accidentally let one of the fluid wells in the MC suck dry = you have just shot yourself in the foot requiring you to start all over again (MC will have gotten filled with air again) DOH! The front brakes will typically start dripping quite quickly as they are much closer to the MC. Once the fronts are dripping steadily, go ahead and close up the front bleeders, then allow the rear to catch up. Once the rear bleeders are dripping steadily with no air gurgling up and out of the bleeders, go ahead and close up the rear bleeders. Top off the brake fluid levels in the MC (about 5/8” from the tops of the fluid wells), then pop the lid back onto the MC. Hop inside of the vehicle and apply the brakes a few times. If you have gotten lucky, you will find a decently firm pedal at this point, and it’s time to move onto the “pump it up and hold it routine” to finish up the bleeding. If you cannot get any kind of pedal yet, there must still be a bunch of air trapped in the system yet. If this is the case, go ahead and gravity bleed further. If the brakes don’t want to gravity bleed, sometimes you can get them going by SLOWLY depressing and releasing the brake pedal about 2-3” inches from the top with the bleeders open at the wheels. Yes, you will always suck some air back inward as you are releasing the pedal, though it is still a winning game as you will suck in about 20% air and produce 80% fluid. The speed bleeders available on the market help a little bit by discouraging air from getting sucked back in during brake release as they are set up to act like one way check valves. IF the MC will not allow fluid to flow during gravity bleeding, the MC piston may be preloaded (not being allowed to reach a full release of the MC piston against the snap ring on the backside of the MC). Loosening the MC away from the brake booster may get the situation moving along as this will allow the MC piston to fully release, though this will have to be investigated and corrected as needed to allow for a definite full state of MC piston brake release before road use (otherwise the brake will lock up as you start driving the vehicle).

* Advanced brake bleeding problems can drive even the most seasoned and experienced technicians nuts! What do you do when everything is “right” yet it’s still not behaving? Under this circumstance, we typically advise that the ports on the MC be plugged using brass or steel fittings as needed. If you depress the brake pedal with the MC ports plugged, you should expect a rock hard pedal with little pedal travel, as the fluid simply has nowhere to go. If you do not have a rock hard pedal with the MC ports plugged, this indicates that either the MC is still full of air or the MC itself is defective. IF you have a rock hard pedal with the MC ports plugged, then you can consider the MC OK and should next only reconnect the front brake circuit. If the pedal is still good and firm with only the front brakes connected, then you know that the MC and front brakes are not the problem. Next, reconnect the rear brake circuit and see how the brake pedal feels – if it has gone into a bad state, then you have at least isolated the problem to the rear brake circuit (which is usually the case). We then typically suggest that the 3/16” brake lines be disconnected from the rear axle flex hose and port plugs be then installed into the brake flex hose. If you are greeted by a good firm pedal with the flex hose plugged, then you would only connect one of the two rear brakes back up. If your pedal “goes to the floor” with that side of the rear brakes hooked up, then you know that you have isolated where the problem is and can focus your efforts to the specific area. Rear drum brakes are very sensitive to the shoe adjustment, which can radically impact brake pedal height = adjust them up as needed to eliminate excessive travel. Rear disc brake calipers with built in emergency brake function can also be VERY tricky to thoroughly bleed (what with all of the extra small parts inside that can trap air), and also to adjust in general. We have learned that the best method for adjusting the integral e-brake style calipers is to have a person hop inside of the vehicle and pump the pedal up as much as possible, then hold firm pressure against the pedal while repeatedly actuating the e-brake about 25-30 times. This forces the caliper pistons to extend out as far as needed to clamp down on the brake pads, then ratcheting the e-brake repetitiously while the piston is extended under pedal pressure allows the e-brake adjustment to then “catch up” to the location of the extended caliper piston. When the e-brake is adjusted up correctly in this type of caliper, you should not be able to see any air gap between the pads and the rotors with the brakes released. You will also find that you will only need to move the lever on the outside of the caliper about ½ way through its travel before it conclusively clamps down onto the pads.

I don’t want to tell you just how many times we have also discovered that customers have accidentally installed their calipers upside down with the bleeder screws at the BOTTOM of the calipers instead of the tops (!). If the right hand and left hand calipers are accidentally transposed during installation, you can run a 55 gallon drum of brake fluid through the system getting nothing but clear fluid out of the calipers and still not “get a pedal” (as the calipers will never fill with fluid, as the fluid will just keep running out of the bottom of the calipers). Make very certain that all of the bleeder screws are at the tops of the calipers or wheel cylinders!