Having trouble bleeding brakes

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Spetz

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Hi guys,

I recently rebuilt all 4 calipers + replaced the BMC and booster so the brake system was completely dry.

Since then I am attempting to bleed the system but with little luck.

First couple of times I did it using "one man kits".
1st time the brake pedal went to the floor with zero braking.
2nd time the brake pedal went to the floor with minimal braking.

3rd time I had someone pressing the pedal for me while I was opening/closing the bleed valve. The result is the pedal still goes to the floor but there is more braking (still very weak though).

The fluid coming out is clean and without air bubbles though so I am not sure what's going on?
Is there a different procedure to bleeding brakes that are dry? ie, to ensure the calipers are properly filled with brake fluid?


Also when bleeding with a helper, do I open the bleed valve before or after the helper presses the brake pedal?
 
Close all bleeders and have an assistant pump the brake pedal until some pedal pressure is felt, it may take 20-30 times. Put a thin piece of plywood under the pedal so it doesn't quite go all the way to the floor.
Once some resistance at the pedal is felt hold the pedal down and crack the bleeder furthest from the MC usually RR, crack the bleeder before letting the pedal up and repeat a few times, then move to the LR Rf and so on. You may need to do this a few times.
 
Make sure the bleeder is closed each time before teh brake pedal is released, otherwise you suck air back in the caliper/line.

My wife helps me do this. I have her hold the brake pedal at the 3/4 position ( never all the way to the floor).

Press, hold , release , are the terms i yell out as i open and close the bleeders. I have her press the brake pedal, the hold, i open the bleeder and before fluid flow stops, i close the bleeder, then i tell her to release. I repeat until i don't see air at each caliper.


Make sure the master cylinder stays topped up. Some cars like saturn s-series have diagonal circuits, but starting with rr then lr should work. If there is a lot of air, sometimes just let the bleedr drip a while before bleeding with the brake pedal.
 
Have had it happen a few times over the years that air gets trapped at the master cylinder. Fixed it by cracking the lines open at the master cylinder one line at a time while helper had the brake pedal all the way down. Tighten the line back up before the helper releases pedal. Might take a few times cracking the line open to find the spot where it starts seeping fluid.
 
Thanks guys.

The master cylinder was not bench bled but it was not a new unit (it came from another car) and had a tiny bit of brake fluid at the bottom.


Brake fluid has gone to all the calipers and when the helper presses the pedal clean fluid comes out which is what is puzzling me (ie, no bubbles or anything)
 
Sounds like you've got air in the master. Having a bit of fluid in the master is meaningless, you've got to completely bleed the master. This will be messy, but you've got to get the air out of the master or you'll never get this system bled properly.

Put a bunch of shop towels below the master. Ensure the reservoir is full with fresh fluid. Disconnect both of the lines at the master, and using your fingers and/or thumbs, close off the fittings with modest pressure. Your fingers are the one way valve for each circuit to ensure that the master gets properly bled.

Have your assistant press down on the brake pedal, fluid and air will squirt past your fingers. You can't stop it, nor should you. But continue to hold pressure against the fittings. When your assistant lets the pedal up, the master will suck fluid from the reservoir, because you've sealed up the line fittings with the pads of your fingers. Repeat until you get nothing but fluid from those fittings when your assistant presses the brake pedal.

Reconnect the lines and bleed the remainder of the system normally.
 
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I just fought this issue on my race Miata after replacing all the calipers. The issue ended up being a bad caliper that wouldn't "clamp" down all the way so when the other pads were in full contact with the rotor this one was barely touching. Once that caliper was replaced everything was fine.

The way to diag this cause is to go to each wheel and try to move the pad carrier that is on the slider pins. If you have a caliper that is not clamping one should move more than the others. If all are good there will be very little movement.

Personally, I'm a huge fan of the Motive Power Bleeders. You can pressurize your system and bleed the MC, ABS, and calipers at the same time. Plus when you have pressure applied you can quickly check that all the brakes are grabbing.
 
You don't say what car. I'm assuming not ABS which I've no experience with but I understand can be a real pain if the system goes dry.

Proceeding on that assumption, I'd try reverse bleeding using a 60ml enema syringe or similar. You can actually cycle fluid in and out which can help to dislodge trapped air.

Some say this can cause trouble by reversing master cylinder seals, but I have used high pressures on my system before I was aware of this alleged risk, without any adverse effect.
 
Thanks guys.

Yes the car is a non-abs version.

I may remove the BMC to try and bench bleed.

Is reverse bleeding an effective way of doing this? I have never heard of it before
 
Originally Posted By: Spetz
Thanks guys.

Yes the car is a non-abs version.

I may remove the BMC to try and bench bleed.

Is reverse bleeding an effective way of doing this? I have never heard of it before


I havn't used it with an assembled-from-dry-components system, but I had a partly drained system due to a leaking wheel cylinder (so the reservoir was empty and the master cylinder presumably at least partly empty) and it got rid of the air on-vehicle without any trouble.

Syringe is cheap, and you don't need an assistant.
 
I replaced the brake lines, calipers and rear wheel cylinders on the Rat. It was a slow process, slowed further by constantly having to check the fluid in the MC, I capped the lines, but they were open as I cut and flared the lines . Drip, drip, drip. If the drips stopped there was an air. Tightened up the line until it was dripping steadily out the open end. I chased the leaks all the way back to the wheel cylinder bleeders. Then I replaced the front lines and hoses. Same process. I finished up with bleeding into an airlock. Whole process was single handed and resulted in a very good pedal. I think I used almost 2 qts of brake fluid.
grin2.gif
 
I would gravity bleed to start. Just open the bleeder on any caliper and the brake fluid will come out. May take a little while. After doing all 4 wheels see if you need to do any additional bleeding with a 2nd person.
 
Reverse bleeding sometimes is pretty awesome because air bubbles, being less dense than fluid, want to go up. You can use a new clean oil squirter and some tubing to make your own reverse bleeder.

Depends on where the bubbles are... I guess. In the calipers, regular bleeding is better provided the bleed is on the top, not on the bottom like on an idiotic BMW clutch slave cylinder. In the lines/master, reverse bleeding works better.
 
Also, be careful not push the brake pedal to the floor as this puts the MC plunger beyond its normal travel range and may score or damage the seals.

I'd also bleed the lines at the MC for good measure.
 
Originally Posted By: andyd
I replaced the brake lines, calipers and rear wheel cylinders on the Rat. It was a slow process, slowed further by constantly having to check the fluid in the MC, I capped the lines, but they were open as I cut and flared the lines . Drip, drip, drip. If the drips stopped there was an air. Tightened up the line until it was dripping steadily out the open end. I chased the leaks all the way back to the wheel cylinder bleeders. Then I replaced the front lines and hoses. Same process. I finished up with bleeding into an airlock. Whole process was single handed and resulted in a very good pedal. I think I used almost 2 qts of brake fluid.
grin2.gif



Brake fluid isn't very expensive, but cycling the fluid using a syringe would avoid most of the consumption in this kind of situation.
 
I called a brake shop today and they suggested that to "bench bleed" the master on the car I can just loosen the outlet lines and press the pedal.

Does this sound reasonable? If I only loosen them a bit would the air be pushed out but not sucked back in?

I do have an 80ml syringe. Would reverse bleeding be a good option? Do I need to follow the "furthest away first" sequence?
 
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