1981 308 brake hydraulic shuttle valve stuck | FerrariChat
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1981 308 brake hydraulic shuttle valve stuck

Discussion in 'Technical Q&A' started by F308fan, Oct 1, 2021.

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  1. F308fan

    F308fan Karting

    May 7, 2017
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    Tony Pacini
    #1 F308fan, Oct 1, 2021
    I just purchased a 308 and the "Brake Failure" warning light has been stuck on. The brakes seem to operate fine. Various posts in this forum led me to remove and check the sensor. The forks were fine (no junk bridging them) so I removed the assembly for inspection. Sure enough, the shuttle valve seems to be stuck to one side (see attached photo).

    Is there a way to repair these, or is replacement the only option?
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  2. theunissenguido

    theunissenguido F1 Rookie
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    Guido
    #2 theunissenguido, Oct 2, 2021
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  3. F308fan

    F308fan Karting

    May 7, 2017
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    #3 F308fan, Oct 5, 2021
    Thanks so much for your reply. I’d seen where someone had used a Volvo valve as a replacement.

    The Volvo valve, though, is slightly different. Capping the unused ports would be easy, but could the large plug on the side of the Volvo valve be replaced with one like the original valve that would accept a brake line?
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  4. theunissenguido

    theunissenguido F1 Rookie
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    #4 theunissenguido, Oct 6, 2021
    To me its the same diameter...and if not, you could use the other port in front instead of that on the side.
     
  5. Ferrari Tech

    Ferrari Tech Formula 3

    Mar 5, 2010
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    Wade Williams
    #5 Ferrari Tech, Oct 7, 2021
    Just pull it apart and clean it up, see if you can get the piston to float again. I have done it several times.
     
  6. raemin

    raemin Formula 3

    Jan 16, 2007
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    #6 raemin, Oct 7, 2021
    Last edited: Oct 7, 2021
    I was trying to fix mine until it came back to life on its own.

    This being said in my quest to fix the plastic plunger that acts as a switch (this is the part that usually fail), I've come across the "Ate Brake Pressure Warning Switch" as used on the Porsche 911 912. This seems the proper diameter, but maybe this is an on/off switch instead off/on, so you may have to reverse the signal with a relay. Normally you can remove the switch from the valve without compromising the circuit seal, so swapping just the switch does not imply to purge the whole circuit.

    You may also just remove the switch and see if this solve your issue. In my case the pressure imbalance had been so high that the switch vertical plastic plunger was stuck by the horizontal metal piston. If it is stuck but not crushed that's an easy fix.

    That's how the whole of it is supposed to work on dual circuits (that's not the Ferrari part, but the diagram is nice) : https://www.mercurycougar.net/threads/can-the-brake-pressure-differential-valve-on-a-67-be-replaced-with-one-from-a-70-73.59952/)
     
  7. F308fan

    F308fan Karting

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    #7 F308fan, Oct 8, 2021
    I did remove it from the car and try to get the spool to move; it appears to be seized. I could see the tab or whatever on the center of the spool (that touches the sensor fork) offset to one side. I tried to move it with a screwdriver and it wouldn’t budge. I didn’t want to soak it in solvent or anything for fear of damaging the seals, so I just reinstalled it for now. I’m very much against the idea of spending $1500+ for an OEM part so I’ll probably have to modify and install a Volvo-style valve or similar.

    When you disassembled it, how did you get it unstuck and what did you clean it with?
     
  8. raemin

    raemin Formula 3

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    #8 raemin, Oct 9, 2021
    I did not disassemble it, I only removed the electric bit. From then on I removed one front bleed screw on the side that had too much pressure and pushed the pedal. So this is the brake fluid pressure that pushed the piston back.
     
  9. waymar

    waymar Formula 3

    Sep 2, 2008
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    Wayne Martin
    #9 waymar, Oct 24, 2021
    FWIW every Spring time I gravity bleed a quart of brake fluid through the system. Never had any issues.
     
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  10. F308fan

    F308fan Karting

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    #10 F308fan, Oct 29, 2021
    Once I get the system working properly I’ll definitely make it a point to run some fluid through it often. Fluid’s cheap. I just got Speed Bleeders that should make that task a bit easier.
     
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  11. F308fan

    F308fan Karting

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    Tony Pacini
    #11 F308fan, Dec 10, 2021
    Last edited: Dec 10, 2021
    Update: I was able to re-center the valve by bleeding. I removed the sensor and used a mirror to see which way the valve was offset (to see which side I needed to bleed). It was toward the rear circuit, so I bled the front and was able to move the valve to the other side (just wanted to verify it wasn’t really stuck). Then I reinstalled the sensor, turned on the key (warning light ‘on’), and bled/pressed pedal until the light went out. Speed bleeders made this into a quick and simple one-person operation. As recommended in another thread, rather than removing the sensor, I could have just opened a bleeder on one end (front or rear), stepped on the pedal with the key and light on, and if that didn’t re-center the valve and turn the light off then I could have closed that bleeder valve, opened the opposite end, and stepped on the pedal again until the light went off.

    As to how it got pushed off center to begin with, the front calipers were rebuilt (and brakes bled) right before I purchased the car, and aside from a leak/failure it’s improper bleeding that usually pushes the valve to one side and turns on the warning light.
     
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