Brake Bleeding Headache

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Brake Bleeding Headache

Postby KenR » Fri Apr 24, 2009 1:29 am

I am looking for some advice on bleeding the brakes, specifically the bugeye single piston rear cylinders....

Scenario:
Using freshly built Bugeye master with 3/4" bore, rebuilt front disc calipers & pads (Apple Hydraulics I think), all new hard lines, new stainless/nylon flex lines, new early style rear wheel cylinders and shoes. I am using 15-18lb pressure at the master cylinder cap to "push" fluid through the lines. Each time I bleed a corner under pressure I take out about 2/3 cup, which is about 3/4 of the reservoir, always being careful not to ever let it go dry. I get the 2/3 cup out in about 25 seconds, so that is a good steady flow which is supposed to be the recipe for carrying bubbles out of the system.


Results: Soft/spongy pedal that almost goes to the floor. At first I had doubts about the master, so I put a fitting and bleed screw at the end of the main hardline (right at the 4-way junction), the result of that experiment was the pedal was hard as a rock. So my air pressure process is good enough to move any bubbles from the high spot/firewall down and out through the 4-way. Second experiment was to re-connect the main line to the 4-way and disconnect the rear line (putting a bleed screw in the 4-way). The object of this was to focus on the front half of the car. Result (after bleeding again)was a little softness and more pedal travel than the plugged main line, but that seemed pretty good and about what I would expect for the entire system in good condition. Then I connected the rear line to the 4-way, tightened the rear brake shoe adjustments, set the parking brake and bled all 4 corners again. Pedal is way soft, almost goes to the floor, but seems much better on 2nd and 3rd pump and holds its position under pressure. No obvious leaks and I did bleed those little auxillary screws/bolts on the top of the caliper near the bleed screw.

Question: As I look at the rear wheel cylinders, it seems quite odd with the bleed on the same 90 degree brass elbow as the hard line. Presumably fluid goes into the wheel cylinder through the banjo bolt (and air comes back out through the banjo bolt) and bleed screw. In theory this should all work...but I really suspect it isn't working in my favor at this time. I usually have a lot of patience, but days and quarts later...I'm thinking that I am forgetting something.
Any suggestions/tips. I have heard of folks just sealing off the rear brake line and going without....but I'm not to that point yet.... :wink:
thanks,
-Ken
KenR
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Re: Brake Bleeding Headache

Postby KenR » Fri Apr 24, 2009 6:47 pm

I think I am answering my own question.... By pushing fluid from the master to the rear bleeder screws, the fluid does not need to pass through the rear brake cylinder. It probably goes from the hard line, into the 90 degree brass fitting and out the bleed screw.... therefore, one would suspect that the air is in the cylinder and I will need to get fluid into and the air out of the cylinder. This is probably best done by getting the piston in the cylinder to move under pressure (pump it up and hold it) and remove the pressure (bleed). So, it seems to make sense that I need to grab a partner and go back to pump it up, push/hold, bleed, stop bleed, let pedal retract, repeat as necessary. With the car up on jack stands, it may also be beneficial to tip the axle up on one side in an effort to get some angle to the horizontal banjo bolt which passes fluid into the cylinder.... Makes me wonder if i could fill the cylinder with fluid on the bench, put a bleed screw in it and have some success with keeping some of the fluid inside while I connected the banjo fitting.
-Ken
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Re: Brake Bleeding Headache

Postby Charlie Tolman » Sat Apr 25, 2009 1:29 am

I have not owned an early Sprite, but I have helped my friend on a few occasions in bleeding brakes on his Mk. I, and we never had the problems that you have described.

A pressure of 15 to 18 pounds per square inch, applied to the master cylinder, seems to be excessive to me. Due to the design of the Mk. I brake line fitting to the rear wheel cylinder, a rapid flow of brake fluid might be blocking the flow of air out of the cylinder, and the fluid then flows directly past the bleed screw and does not enter the cylinder.
I would use the "two person" approach to brake bleeding, where the bleed screw is loosened, the brake pedal is depressed SLOWLY, the bleed screw is tightened, and the brake pedal is returned to its 'up' position, etc.

As you suggested, sometimes it helps to have the corner of the car raised somewhat, to aid in bubble flow through the brake lines.

Out of curiosity, why do you set the parking brake before bleeding the rear brakes?

Charlie T.
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Re: Brake Bleeding Headache

Postby Charlie Tolman » Sat Apr 25, 2009 10:59 am

I mentioned your problem to a friend, and here is his reply:

Nothing wrong with the two person method. I think 15 to 18 is probably too much and he's opening the slave too much as well. This is a process that doesn't work well if you're rushing it.

Another way to do it is to fill the master, crack open the slave cylinder with a tube down into a receiving vessel, and let it "self bleed" very slowly for a while. No pumping, just make sure the level in the master doesn't get too low and take air into the lines. I've done it on the Mini and it does work just fine. If you have a clear tube on the slave, you'll see the air bubbles coming through.

Charlie T.
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Re: Brake Bleeding Headache

Postby KenR » Sat Apr 25, 2009 7:10 pm

Thanks All, It is pretty strange design, with the MK I slave oriented at 45 degrees with the pressure line in/exit line out located at the bottom. I'll try the 2 person method and slowly... The pressure method was primarily used to get the air bubbles out fo the high area along the firewall and down hill out of the 4-way. It worked fine for that, but as you suggest... probably isnt the right approach for the rear slave cylinders. I had a nightmare last night where I had to remoe the piston out of the slave, fill with fluid, re-install and then put the shoes back on.... I'd rather not go down that path.....
thanks,
-Ken
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Re: Brake Bleeding Headache

Postby Charlie Tolman » Wed Apr 29, 2009 12:51 am

Fluid flow, liquid entering and/or air leaving, through the strange brass fitting, is compromised by the banjo bolt. There usually are two small holes in the banjo bolt near its hex head, and a small diameter passage through the length of the bolt.

With a moderate or high flow of brake fluid into the banjo bolt during a bleeding operation, it would be difficult for air in the brake cylinder to flow out of the cylinder against the incoming flow or pressure of brake fluid.

The "gravity" method might be the best method, as recommended earlier in this forum.

Charlie T.
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Re: Brake Bleeding Headache

Postby KenR » Wed Apr 29, 2009 11:25 am

Right, I agree with the "slow" movement of fluid probably being better. I am really hooked on the port into the side of the cylinder being at the bottom of the arrangement. In theory, bubbles would be at the top with pressure/bleed port at the bottom...just doesn't make sense. I suppose I could pull the axle assembly out of the car, stand it on end and fill the bottom most cylinder with a syringe, fill the hard line and assemble them with a capped off hard line, flilp the axle on the other end, fill cylinder with syringe, fill hard line and assemble with capped off hard line, put the axle back in the car, connect hard lines to flex line and bleed from there. Maybe one could fill the cylinders with fluid on the bench, put a plug in the banjo bolt hole and re-fit the brake cylinders to the backing plate with the plug in place...then remove plug and attach the brass 90 degree with hard line and bleed from there....
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Re: Brake Bleeding Headache

Postby Frogbird » Sat May 06, 2017 8:16 pm

Did you ever solve this problem?
I have the identical situation. New everything including 3/4 bore master cylinder and nothing I do imprioves the pedal.
I have bled the brakes every way now to man and remains the same.

Thanks
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