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  1. #1

    Default Brake system rebuild - now no brakes

    Having never replaced the rubber brake hoses and master cylinder on my 85 GT, I figured it to be a good time, being somewhat fearful of a master cylinder failure while driving (happened to a friend with his 65 GT350).


    SoI replaced the front hoses and the rear hose at the T-intersection, plus the steel lines from the firewall back and out to the rear wheels, and the master cylinder. Rebuilt the rear drums. Fully bled the system using a vacuum bleeder. The emergency brake cable lever has resistance when pulling back on it. However, I cannot get any pressure in the brake pedal and the brake light is on (engine running). There are no drips or leaks; fittings are dry.


    Can the new master cylinder be bad? Ive read that one can disconnect the brake lines at the master cylinder, plus the outlets, and test it that way. Seems like it would work, but is there any way to check it out that does not involve opening the system and then bleeding again?


    What else can be wrong?

    [Hope this displays correctly, the web site isn't loading properly for me]
    85 GT (original owner)

  2. #2
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    qikgts's Avatar
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    What was the condition of the fluid in the master when you started this project? If it was kinda nasty you may need to disassemble/clean/reseal the proportioning valve.

    https://www.musclecarresearch.com/va...-E0ZC-2B328-AA

    I say that because you mentioned the light is on now and it wasn't before. The light only comes on if:

    1- The "microswitch" near the parking brake handle is tripped
    2- If the switch in the prop valve senses no pressure on the front or rear side
    3- If any of the light wiring has been shorted
    '85 GT

  3. #3

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    Did you bench bleed the MC before you installed it?

  4. #4

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    Did you bench bleed the new master cylinder before you put it in?

  5. #5

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    Quote Originally Posted by 1984GTTurboVert View Post
    Did you bench bleed the new master cylinder before you put it in?
    I did bench bleed the master cylinder. Seemed to check out fine.
    85 GT (original owner)

  6. #6

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    Quote Originally Posted by qikgts View Post
    What was the condition of the fluid in the master when you started this project? If it was kinda nasty you may need to disassemble/clean/reseal the proportioning valve.

    https://www.musclecarresearch.com/va...-E0ZC-2B328-AA

    I say that because you mentioned the light is on now and it wasn't before. The light only comes on if:

    1- The "microswitch" near the parking brake handle is tripped
    2- If the switch in the prop valve senses no pressure on the front or rear side
    3- If any of the light wiring has been shorted

    The master cylinder is new and has only had new fluid in it.

    The brake light may have come on when I had not tightened the brake lines completely and lost pressure. I saw the leaks and now everything is tight, but I have no pedal pressure. It returns to the up position when I take my foot off.

    Does the "microswitch" near the parking brake handle need to be reset somehow?

    I suspect reason no. 2 is why the light is on; I have no pedal pressure.

    Still wondering if a new master cylinder can be bad.
    85 GT (original owner)

  7. #7

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    I think the shuttle valve in the prop valve needs to be centered. this is why the light is on.
    What I found with a quick google search is to have an assistant slowly depress the brake pedal while cracking the rear bleeder and close it when the light goes out. On the older cars there is a tool that replaces the switch for bleeding that holds the shuttle valve centered, and when finished you replace the switch.
    Mike
    85 GT - owned since 87

  8. #8
    FEP Super Member gr79's Avatar
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    System rebled after leaks were tightened?

    I would rebleed only the master cylinder first if no bleed kit was used.

    Insure brake fluid in the m/c bowls, clear bottle, quart jar, or catch can, does not run dry and expose the open ends of tubing to air while pumping.
    Insure tubes stay submerged in brake fluid when they may wiggle when stroke reverses from pump to suck during manual pump stroke bleeding.

    Use couple feet of clear vinyl tubing to watch flow and for any stubborn air bubbles and slugs.
    Clear jars and long vinyl tubing work every time here. Will use up a quart of brake fluid to get firm brakes again.
    Bit of teflon tape on bleeder screw to keep air from entering thru the threads of the open bleeder screw during bleeding.
    Turning the red brake light out is another story cant help. Older cars brake check light system is simpler than newer.
    Got front calipers and hoses to do here. Needed to refresh memory and mood to get that done this year.

    The Ranger's hyd clutch bleed is similar to brake bleeding but procedure is not a job for me.
    Trans shop could not get it exactly right after a clutch/brg replacement. Dealer did.
    Last edited by gr79; 07-19-2023 at 01:29 PM.

  9. #9

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    Quote Originally Posted by gr79 View Post
    System rebled after leaks were tightened?

    I would rebleed only the master cylinder first if no bleed kit was used.

    Insure brake fluid in the m/c bowls, clear bottle, quart jar, or catch can, does not run dry and expose the open ends of tubing to air while pumping.
    Insure tubes stay submerged in brake fluid when they may wiggle when stroke reverses from pump to suck during manual pump stroke bleeding.

    Use couple feet of clear vinyl tubing to watch flow and for any stubborn air bubbles and slugs.
    Clear jars and long vinyl tubing work every time here. Will use up a quart of brake fluid to get firm brakes again.
    Bit of teflon tape on bleeder screw to keep air from entering thru the threads of the open bleeder screw during bleeding.
    Turning the red brake light out is another story cant help. Older cars brake check light system is simpler than newer.
    Got front calipers and hoses to do here. Needed to refresh memory and mood to get that done this year.
    I rebled all four corners after fixing the leaks.

    I did use a bleed kit to set up the m/c, and a vacuum bleeder for the brakes, using wheel bearing grease to seal around each bleed screw. Was able to get solid fluid flow with no bubbles. Tubes and m/c bowls remained submerged and filled with fluid during bleeding. I didn't think about teflon tape; good idea, and less messy. The vacuum bleeder came with a bottle to attach to the m/c and keeps the bowls filled automatically.

    The front brake hoses were probably the easiest part of the project for me. It was access to the hose at the T-intersection above the differential that was...a challenge.

    I would think the brake light will go out once I get pressure in the system, so it (the light) is not a big concern.

    Should nothing I missed come to light I may just start from the beginning again, starting with the master cylinder.
    85 GT (original owner)

  10. #10

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    Quote Originally Posted by JKengineering View Post
    I think the shuttle valve in the prop valve needs to be centered. this is why the light is on.
    What I found with a quick google search is to have an assistant slowly depress the brake pedal while cracking the rear bleeder and close it when the light goes out. On the older cars there is a tool that replaces the switch for bleeding that holds the shuttle valve centered, and when finished you replace the switch.
    Mike
    What is this switch you're referring to?
    85 GT (original owner)

  11. #11
    FEP Senior Member
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    Post #7 in this thread has some information on what may have happened:
    http://vb.foureyedpride.com/showthre...Lug-Conversion
    '89 XR-7 5 Speed
    '95 SC 5 Speed
    '91 Crown Vic P72 351W
    '97 Thunderbird
    '85 Ford LTD Squire

  12. #12

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    Quote Originally Posted by KevinVarnes View Post
    Post #7 in this thread has some information on what may have happened:
    http://vb.foureyedpride.com/showthre...Lug-Conversion
    Okay, that helps. I also found the relevant section in the shop manual talking about this. I need to make myself smart on this before heading out to the garage again.
    85 GT (original owner)

  13. #13

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    As I understand the Pressure Differential Valve (PDV), the piston will shift to one side if it senses a loss of pressure in either the front or rear brakes. The PDV will shut off fluid flow to the bad side and let the good side to continue to function. If the rear brakes failed I would still have front brakes.

    As fluid flow to the rear brakes is shut off I would not be able to bleed the rear brakes, but that is not the case. I'm getting fluid flow and solid (no air) flow on all four corners, and the master cylinder fluid needs to be topped off during the bleeding.

    The light is on since there is no pressure at all in the pedal.

    However, the manual does say to centralize the PDV after bleeding the system, but it doesn't say how to do so. Is this what I'm missing?

    Sorry to be a knucklehead on this, but the car's brakes have never been apart (other than brake pads).
    85 GT (original owner)

  14. #14

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    I would start by cracking fittings and seeing where you have pressure and where you don't. Brake fluid is very corrosive and will eat paint, so use a rag to protect stuff you might need a helper to pump the pedal and hold it while you crack fittings with a rag handy.
    No pressure at the pedal points to master cylinder since it is the first link in the chain. Then just work your way from the master, cracking fittings along the way to see where the problem is?
    just my opinion
    i have had brake problems in the past, very frustrating.
    79 Zephyr, 4.6L 4v/4r70w swap, with team z front and rear suspension, 8.8 and upgraded brakes and coil overs. Running Holley Terminator X Max.

  15. #15
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    Quote Originally Posted by Luke85 View Post
    However, the manual does say to centralize the PDV after bleeding the system, but it doesn't say how to do so. Is this what I'm missing?
    https://www.musclecarresearch.com/brake-valve-tool
    '85 GT

  16. #16

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    I finally got the brake system completed (I left it alone for a while), with the last two issues resolved with the help of a mechanic friend. This after a bench bleed of the master cylinder and a successful bleed at all four corners.

    1) Soft brake pedal with engine running. Once the system has been completely assembled and bled I had to bleed the master cylinder at each of the two outlet ports. Using a constant brake pedal pressure, crack open one line at a time to let out any trapped air at this location. He said he ALWAYS had to do this.

    2) Brake light on. Re-centering the Pressure Differential Valve (PDV) was done by repeatedly pressing hard on the brake pedal with the engine running. After a half dozen or so HARD pushes, the light went out.

    I came across neither of these methods in my search of the manual or internet. Submitted in case anyone reading should ever have to go through this.
    85 GT (original owner)

  17. #17
    FEP Senior Member Patrick Olsen's Avatar
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    Sucks that it was so painful for you to get it all worked out, but thanks for the follow-up.

  18. #18
    FEP Super Member gr79's Avatar
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    When working in a auto parts store, learned seeing a MC being bench bled first time. A MC bleed kit was new to me.
    Customers were asked if they wanted that done for free.
    MC cover was reinstalled and outlets capped for the ride to repair site.

    Hold MC down on bench or vice. Remove cap, leave off for simple bench bleed.
    Fill MC with brake fluid. Keep MC full or at least the holes to the internals visuably full.
    Connect tube nuts with tubing securely to MC outlet ports. Loop tubes back into MC resouvour.
    Pump MC with screwdriver until fluid flows steady back into MC. Done.
    Clear tubing works best to see air bubbles and fluid flow.

    I always push hard at the end to test brakes. Aint gonna break anything that brake pedal has to hold.
    Did not know doing that would center a non anti lock PDV valve and turn off light.

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