Brake booster problem

[Bruce Compton, Canada]

I have a Midland remote booster (55-57 T-Bird type) on my '54 Merc and it somehow builds up pressure to the extent that the brakes almost lock. This happens when the car is left running but parked. Can anyone help explain what's happening?? Thanks : Bruce

[Jwawhite]

What about the valve between vacuum line and booster?

[miker98038]

It’s been a lot of years since I’ve been around one of those, but in general a locking booster under heat is a lack of clearance between the internal rod from the vacumn section to the master cylinder. Parked and idling, the heat builds up, everything expands, and the brakes get applied. If I could find an exploded diagram I could be more specific.

[KULTULZ]

Originally Posted by miker98038

It’s been a lot of years since I’ve been around one of those, but in general a locking booster under heat is a lack of clearance between the internal rod from the vacumn section to the master cylinder.

Parked and idling, the heat builds up, everything expands, and the brakes get applied.

If I could find an exploded diagram I could be more specific.

Here 'ya go-

[miker98038]

I’d forgotten how complicated those were. No wonder I’m using a hydro boost. Somewhere in the stack of parts starting with rod 2377, there’s got to be an adjustment for free play between it (or something in the middle) and where it acts on the piston of the “boosted master cylinder “ 2425. Somebody else is going to have to help out, I just can’t recall how or if there was an easy adjustment. I suppose it could also be a lack of free play on the pedal/firewall cylinder connection. Anything not having some free play to allow for heat expansion would cause the brake application. I’ve had that with more typical firewall boosters as well as the hydro boost units. Either pedal to booster or booster to cylinder.

[KULTULZ]

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I have a 57 SHOP MANUAL here somewhere... ...that describes the system (still used in BIRD).

Interestingly, the 55/57 BIRD used a heat shield @ the MC as I assume the tighter engine compartment allowed engine heat absorption, leading to the same malady.

Let me dig tomorrow and I will see if I can find it.

[KULTULZ]

Originally Posted by Jwawhite

What about the valve between vacuum line and booster?

That is a vacuum check valve (2365 - shown in the Parts Ill). It opens while the engine is running to allow vacuum draw to the booster assy. It closes @ engine shutdown to keep a vacuum reserve inside of the booster reservoir in case the engine shuts off and you need the booster assist to stop the car.

It also prevents an engine backfire from entering the reservoir. If it sticks open (fails), no vacuum can be held in the reservoir.

Good point though as usually the rubber hose(s) will deteriorate over time losing the signal. There is also a filter in these things that needs to be changed regularly.

[Alaska Jim]

also check the length of the rod from the brake pedal to the firewall side of the booster. If it is adjustable, and has ever been played with , it can cause this problem

[Bruce Compton, Canada]

Thanks guys, but this booster has performed perfectly for three years and only started acting up this winter in below freezing temperatures when I was putting cars away for the winter. After saying this, I can't see excess heat causing a problem and can't understand how a rod could increase in length and apply the brakes. I'm still confused so keep the suggestions coming . Thanks again : Bruce

[miker98038]

If it was freezing and working before, my idea is out the window. But lack of clearance in a hydraulic system will allow enough expansion from heat to apply pressure. Been there.