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Thread: Pressure Bleeding Problem

  1. #1
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    Pressure Bleeding Problem

    We ran into a problem pressure bleeding the brake system yesterday. I have the CNC reservoir with the pressure bleeding cap. Fill the reservoir with fluid, pressurize it with a bike pump, open the bleeders until clear fluid, no bubbles is coming out. Close the bleeders and the pedal is really solid. All good so far. Then released the pressure from the reservoir, and the pedal is really soft and goes all the way to the floor. Still plenty of fluid in the reservoir at this point. We sent a pretty good amount of fluid through the system so I have to think we got the air out. Any ideas as to what might be causing this?

  2. #2
    Papa's Avatar
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    Did you bench bleed the master cylinders?
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    Seasoned Citizen NAZ's Avatar
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    First the basics:
    Ensure the M/Cs return all the way to the stop so that the vent port is uncovered and that the bottom of your reservoirs are above the M/C feed ports. And of course, the hoses coupling the reservoirs and the M/Cs should not be higher than the bottom of the reservoirs -- the fluid doesn't run uphill very well unless under pressure. The reservoirs must be vented to atmosphere but that vent should have an elastomeric diaphragm between atmosphere and the fluid to prevent the fluid from absorbing water from the air.
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    Senior Member Avalanche325's Avatar
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    Does your pump stay attached to the cap? If not, there is hardly any air volume.

    If you didn't bench bleed, the Wilwood M/Cs do seem to bleed OK in the car. I just did a hydraulic clutch upgrade and didn't bench bleed because it was a completed car. I had to pump the pedal a bit, but it wasn't too bad. To get the M/C to fill, I find that partial 1/4 - 1/2 strokes work better than going the floor.

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    Papa's Avatar
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    When I say bench bleed, I mean in the car. Just disconnect the outputs from the master cylinders and connect short pieces of tubing that run from those outputs and dump into the reservoir. Fill the reservoir with fluid and pump the brake pedal until it's flowing all fluid, no air bubbles. Reconnect the brake lines and then pressure bleed the lines and calipers.
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    Thanks for the input.

    Papa- I did not bench bleed, I'll give that a try as you suggest.

    NAZ- I'll check those points too, but I do think we have the reservoir as the highest point in the system.

    Avalanche- pump stays connected to the cap during the process, there is an audible hiss of the pressure being released when you unscrew the cap. I might try your method of 1/4 to 1/2 pumps to see if we can get the MC filled before I pull off the output lines to follow Papa's method. Should we have a bleeder valve open during the pumps or have the system totally closed? I'm thinking we would need to have something open to allow for flow into the system

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    Senior Member edwardb's Avatar
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    I've used the CNC pressure cap method on four builds and it hasn't let me down yet. Full disclosure, my Coupe build has Tilton reservoirs. So bought a spare Tilton cap and put a Schrader valve on it mimicking the CNC cap. Also worked fine. My process is I bench bleed the masters exactly as Papa described. Once that's done, fill the reservoir leaving some airspace. Then pressurize. I use my compressor with the regulator turned way down. To 5 PSI or so. A bicycle pump should be the same. I guess it doesn't hurt to leave the attached. But not necessary for me because I use a tire air chuck. The cap shouldn't be leaking, e.g. hissing as described. Figure out why and fix it. Agree the pressure will be used up pretty quickly, especially the first time through when getting all the air out. But no big deal. Just hit it with some air pressure a couple times max on each caliper. Never pump the pedal during the process. Once you start, save the pedal press until you're done to confirm a hard pedal. With pressure in the system, pushing the pedal could damage seals. The one thing I see missing from all the descriptions is the order you bleed is important. Always the farthest first, then in descending order. So right rear, left rear, right front, left front. No reason to open the bleeder until you're at the caliper. I put a clear plastic tube on the bleeder and have the end in a small jar with some fluid. Open the bleeder and run until bubbles stop. May be necessary to close the bleeder, re-pressurize, and go again. Even a couple times the first time through. Make sure the bleeder is at the top of the caliper, and if using Wilwoods where they sometimes have two, always the top one. I go all around in the order mentioned. Once done, then around a second time. Usually this just confirms there are no more bubbles. But just to be sure. This process has always resulted in a hard pedal for me. I've found the process also works fine for a hydraulic clutch.
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    Senior Member CraigS's Avatar
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    I have CNC MCs. When I was talking to them they gave me two warnings. 1- use a max of 5#. This could be a problem for you. Since there is just a tiny bit of air above the fluid in the MC, I suspect that you are way over the 5# max. 2- never move the brake pedal w/ the pressure on. I am not sure of the reason for this but they were adamant.
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    Senior Member rich grsc's Avatar
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    Quote Originally Posted by chjort View Post
    We ran into a problem pressure bleeding the brake system yesterday. I have the CNC reservoir with the pressure bleeding cap. Fill the reservoir with fluid, pressurize it with a bike pump, open the bleeders until clear fluid, no bubbles is coming out. Close the bleeders and the pedal is really solid. All good so far. Then released the pressure from the reservoir, and the pedal is really soft and goes all the way to the floor. Still plenty of fluid in the reservoir at this point. We sent a pretty good amount of fluid through the system so I have to think we got the air out. Any ideas as to what might be causing this?
    You should not press the pedal with pressure applied, the air pressure can prevent the piston from returning, or even force fluid behind the piston.

  10. #10
    Seasoned Citizen NAZ's Avatar
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    After half a century of working on a variety of brake systems I’ve used several methods of bleeding brakes and juice clutches. All work – some are just easier. Since I did this work for a paycheck, I had to be able to quickly bleed brakes by myself.


    I prefer to use the vacuum method as it’s not as cumbersome as pressure bleeding. You still use pressure to force the fluid through the system but it’s atmospheric pressure not compressed air. The vacuum method uses a see-through container between the vacuum source and the caliper. Pull a vacuum on the caliper you want to bleed and control the flow with the bleeder valve then simply watch for air bubbles while keeping the M/C reservoir full. Since you don’t need a cap on the M/C reservoir the process is fast and easy. My first vacuum pump was made from an old Ford mechanical fuel pump but years ago I discovered the MityVac and been using it ever since.

    I also don’t bench bleed the M/C – too much time and work. Bench bleeding seems to have become a mythical “requirement” for some but I’ve never come across a M/C that “required” bench bleeding. The real advantage of bench bleeding a M/C is pushing a bunch of air out of the M/C that would have had to been pushed through the brake system. Let’s take a typical Wilwood 3/4” diameter GS compact remote M/C like FFR may supply in a kit. That M/C displaces .55 cubic inches and the first stroke full of air will fill more than 40” of typical 3/16” brake line with that air. That can make using the old-fashioned pumping the pedal method harder but if you are power bleeding your system (compressed air or vacuum) the most it will do is use a bit more fluid if the brake lines already have fluid in them.


    But like I said, all the popular methods of bleeding brakes work so find one you like and use it.
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  11. #11
    Senior Member Avalanche325's Avatar
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    Quote Originally Posted by chjort View Post
    I might try your method of 1/4 to 1/2 pumps to see if we can get the MC filled before I pull off the output lines to follow Papa's method. Should we have a bleeder valve open during the pumps or have the system totally closed? I'm thinking we would need to have something open to allow for flow into the system
    Bleeders closed, no pressure, actually reservoir caps / diaphragms off. Go nice and easy. I just added a clutch M/C and it worked fine. If not, go to Papas method. Then pressure bleed the whole system.

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