70GS 455 Brakes Hold Off Valve

Discussion in 'The whoa and the sway.' started by tminihan, Apr 8, 2020.

  1. tminihan

    tminihan 1970 GS455 Vert - Project

    70GS 455 with power brakes, disc front, drum rear, prop valve and separate hold off valve. Generally the brakes are adequate but don't feel great. Moderately stiff pedal, car occasionally creeps a little while at a stop. I have little knowledge of the engine but it appears to run well however I have 16" vacuum at the booster, which I understand is marginal. I believe the engine had some performance mods when rebuilt by the PO.

    I will be off work next week so I plan to tackle this to rule out everything but too low vacuum. I purchased the car disassembled so I don't have "before" braking performance. The car has new booster, master cylinder, prop valve, hold off valve, calipers, pads/shoes, wheel cylinders and lines.

    1) I did not have any issues bleeding but I did not do anything with the hold off valve. I read on this forum I should have depressed the plunger before bleeding, Do I need to hold the plunger in while bleeding or do I just press it once? How much motion will it have?

    2) The pedal does drop approx 1" (swag) when I start the engine, check valve seems fine, and the booster seems to hold a vacuum well after the engine is shut off, so I feel like the booster is fine.

    3) Is there a way to attribute this to too low vacuum other than by process of elimination?

    4) I spent a couple of hours today reviewing the "hard pedal" threads, any other thoughts are welcome.

    (After this posting I found Larry's post about using a C-clamp on the hold off valve).

    Thanks, Tom
    Last edited: Apr 8, 2020
  2. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD"

    I have 12" of vacuum in gear and my brakes are fine. 16" is plenty, not marginal. According to the Chassis Manual, you need to hold the valve in through the entire bleeding process. This is for 1971 models with the combination valve, but it applies to the separate hold off valve as well.

  3. tminihan

    tminihan 1970 GS455 Vert - Project

    Thanks Larry. I tried to hold in the pin on the hold off valve and it wouldn't budge. Pretty tight area for a c -clamp so I pulled the valve. It took a two handed pull on my bench vice to break the pin loose. Upon disassembly, there was nothing but rust and solids. I'm hoping this was my problem. I ordered a new valve from Inline, hoping it comes this week before I return to work. The story of my life, I would have replaced/rebuilt the engine to improve the vacuum only to learn that it was the $100 hold off valve. The interesting part is that I didn't have problems bleeding about a year ago - but I typically gravity bleed. Thanks for the procedures, I should know by this weekend if the new valve corrected the problem or additional troubleshooting needed. Thanks, Tom
  4. gstewart

    gstewart Well-Known Member

    I have a similar problem with the prop valve on my '72 GS. I was able to depress & hold the pin in. However when the bleeding was completed there was no way to get the pin to release. Bought a new one. I disassembled it but I kept it. I think it is rebuildable.

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