Brake Bleeding Issue-66 Manual Drum Brakes

highhorseb · Jul 31, 2007

We're having trouble getting all the air out of the brake system on a 66 manual drum brake car. We replaced the master cyl, vent hose on the rear axle and the brake pads. Wheel cyl are not leaking and no visible leaks on any lines or fittings. Is it possible that we are sucking air through the rear brake vent hose on the rear axle. Is that a one way valve in there? I bleed each wheel starting from right rear, left rear, right front left, left front until no more air bubbles are visible. Pedal will firm up temporarily but will quickly give out and go to the floor. Bleed again and come up with the same issue. We've attempted this 6 times to no avail. Any help would be appreciated.

Capt Dan · Jul 31, 2007

I believe the vent hose vents the axle, not the brakes.

If you replaced the MC you may have to purge more than 6 times. Probably need to really bleed the system from the passenger rear to get all the air out. Someone else who has done it may be able to better answer.

66HertzClone · Jul 31, 2007

Despite rumors to the contrary, it is a must to bench bled a master cylinder prior to installation. Remove the master, clamp it level in a vise, connect a line to the outlet and submerge it in the fluid in the reservoir. I use a #3 or larger Phillips screwdriver to push the piston in, do this a slow and as evenly as possible. When no more bubbles appear, let it still untouched for a few minutes, I usually tap the master with a hammer to release any small air bubbles that might be clinging to the internal parts. I then cycle the piston a few more times to remove any remaining air.

Leave the line connected if possible while you mount the master back in the car., once removed air could again enter the master cylinder. Bleed at all of the wheels starting at the furthest from the master ending at the left front.

highhorseb · Jul 31, 2007

66HertzClone said:
Despite rumors to the contrary, it is a must to bench bled a master cylinder prior to installation. Remove the master, clamp it level in a vise, connect a line to the outlet and submerge it in the fluid in the reservoir. I use a #3 or larger Phillips screwdriver to push the piston in, do this a slow and as evenly as possible. When no more bubbles appear, let it still untouched for a few minutes, I usually tap the master with a hammer to release any small air bubbles that might be clinging to the internal parts. I then cycle the piston a few more times to remove any remaining air.

Leave the line connected if possible while you mount the master back in the car., once removed air could again enter the master cylinder. Bleed at all of the wheels starting at the furthest from the master ending at the left front.

Correct me if I'm wrong, but this process could be done in the car if I'm reading correctly..we would just need to remove the brake line for another and run it into the reservoir, pump the pedal slowly inside the car until airbubbles are no longer present, then hook it back up to the system. Also, when removing the bleeder line from the MC, what will keep the fluid in while we hook up the MC to the cars brake line?

66HertzClone · Jul 31, 2007

The key to getting this done is making sure the master is level. If your car is not level, its just not going to work. I always bled the master before installing it in the car. Get a new short section of bendable steel line from the parts store, use it for the bleeding process. Sometimes a master will include a plastic fitting and a short section of rubber hose for bleeding it, find anything like that in the box?

highhorseb · Jul 31, 2007

66HertzClone said:
The key to getting this done is making sure the master is level. If your car is not level, its just not going to work. I always bled the master before installing it in the car. Get a new short section of bendable steel line from the parts store, use it for the bleeding process. Sometimes a master will include a plastic fitting and a short section of rubber hose for bleeding it, find anything like that in the box?

Neither of these were included nor have they been on the ones I've previously installed (never had this problem with other vehicles, 69FB, 65 cpe, 95GT, 89GT). I guess I'm just having a problem with the concept of bench bleeding and what difference it makes if its level, bled in the car, out of car, hooked to the braking system or not. As long as the MC never runs out of fluid, eventually, the air should (and has in the past) come out, at least in my small level of thinking. And from what I'm thinking with bench bleeding, if we have fluid in the MC, as soon as I disconnect the bleeder line, some if not all of the fluid will run out.

BullittStangV8 · Jul 31, 2007

heres a simple..yet dumb question. Does that master cylinder "lid" twist off (65-66), or lift/pop off? I for the life of me cannot get mine off. You would think after building a long block something like this would be simple!

highhorseb · Jul 31, 2007

Screw off. I meant the cap, not in a derogatory way..

BullittStangV8 · Jul 31, 2007

haha thanks.

66HertzClone · Jul 31, 2007

highhorseb said:
Neither of these were included nor have they been on the ones I've previously installed (never had this problem with other vehicles, 69FB, 65 cpe, 95GT, 89GT). I guess I'm just having a problem with the concept of bench bleeding and what difference it makes if its level, bled in the car, out of car, hooked to the braking system or not. As long as the MC never runs out of fluid, eventually, the air should (and has in the past) come out, at least in my small level of thinking. And from what I'm thinking with bench bleeding, if we have fluid in the MC, as soon as I disconnect the bleeder line, some if not all of the fluid will run out.

Try doing a Google search for bleeding brakes......

here are few examples of what I found, notice everyone says how important it is to bleed the master cylinder.

One

Two

Three

Four

Five

No doubt, you could eventually purge the air from the master without bench bleeding, your previous experience shows that is possible. However, your opening statement makes it pretty clear that's not working this time. If the master is not on a level plane when trying to bleed it, air bubbles may become trapped. Brake fluid does have some flow resistance, thicker than water and the bubbles tend to cling. The piston in the master has a few seals and the edges and lips give the air a good place to hide.

You are correct, as soon as you loosen the nut, fitting whatever, fluid will leak out and air will take its place. But, not along the length of the piston or in the bore of the master. Once connected and you bleed at a wheel, the movement of fluid will push these air bubbles down the line and out the bleeder screw.

highhorseb · Jul 31, 2007

66HertzClone said:
Try doing a Google search for bleeding brakes......

here are few examples of what I found, notice everyone says how important it is to bleed the master cylinder.

One

Two

Three

Four

Five

No doubt, you could eventually purge the air from the master without bench bleeding, your previous experience shows that is possible. However, your opening statement makes it pretty clear that's not working this time. If the master is not on a level plane when trying to bleed it, air bubbles may become trapped. Brake fluid does have some flow resistance, thicker than water and the bubbles tend to cling. The piston in the master has a few seals and the edges and lips give the air a good place to hide.

You are correct, as soon as you loosen the nut, fitting whatever, fluid will leak out and air will take its place. But, not along the length of the piston or in the bore of the master. Once connected and you bleed at a wheel, the movement of fluid will push these air bubbles down the line and out the bleeder screw.

I appreciate the help, and wasn't attempting to say you were wrong and I was correct..just trying to justify the difference in my mind. I will take a gander at the information on the links. Thanks again!

66HertzClone · Jul 31, 2007

Understood, and I hope that you didn't think I was. I am still having problems wrapping my head around some front end alignment issues I'm having with my car.

dennis112 · Jul 31, 2007

Take the line off at the master cylinder and put your thumb over the hole. Have someone push the brake pedal and see if it squirts. If not, keep your thumb over the hole and have them push the brake again. Repeat until it squirts and then quickly install the brake line to the MC. Yes it is a little messy and be sure to have your safety glasses on, plus a rag to clean up the mess. Never let brake fluid remain on painted surfaces.

Next bleed that line at the MC by loosening it up a half turn and have your helper push the pedal to the floor. Tighten the line, then have her release the brake. Repeat until you get a good squirt, just like it should at the wheels.

Then bleed the wheels as normal.

Brake Bleeding Issue-66 Manual Drum Brakes

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