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I have examined three master cylinder reservoir caps and they all have a pin hole in one of the four corners at the top of the cap. Is this to allow tiny bit of air so the reservoir refills with fluid after braking?

Yes. The reservoirs are vented open to air on the regular BTV. If the car has had a remote fill added and a regular or an aftermarket replacement cap was drilled and threaded for a hose fitting then the vent hole must be plugged.

One of the forum posters recently found a cap that fits which might be worth looking into. It has a built in bladder to allow the fluid expansion etc . I think he posted the info on the forum late last year. One of the issues with the old open air venting system is since non silicone based fluids collect moisture the fluid deteriorates and needs to be changed on a regular basis.

Most of our old Packards rarely see the fluid changes so the fluid becomes gummy or even has hard crystals develop and float around. Those hard crystals do not play well with the BTV compensating valve and I suspect could be part of the issue where suddenly there are no brakes at one stop but everything is OK on the next. If a crystal landed on the valve seat preventing a tight seal the fluid is forced back into the reservior instead of out to the wheels.

Here is the post:

Bladder-style cap for treadle-vac

Thank you HH56 and JeromeSolberg for your responses. I think I may add a remote fill to my 400 to eliminate the atmospheric condition.

All the best!

Here is what I did for a remote reservoir

One remote reservoir option

And here is another one

Another remote reservoir option

Thanks Jerome for providing the options. I had been following that thread myself and found the installations fo be very well done.

Thanks Jerome for the options on remote reservoirs. I just ordered the Lincoln kit from Hwy 101. I'll report back when I have time to install it.

I Installed the remote reservoir I bought from Highway 101 Rod and Custom in WA. It says it's for a Lincoln but it fits perfectly on the BTV. Attached are a few pictures, I am very pleased with the look.

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  PXL_20220205_211930959.jpg (286.67 KB)