[Dave in MN]

Woofa...
I suggest you remove the radiator and take it to a qualified radiator repair shop. This is your only cost effective solution to end up with the best result. Oh...and keep the 4# cap! You bought the sealed system after careful consideration so trust yourself. Use it. Good luck with the repair.

I take issue with those who say throw the 4# cap away.
I use a sealed system with a 4# cap on two of my cars I tour with. The first and longest installed system in my Phaeton has over 96,000 miles on the sealed system. (Third water pump though...bearings wore out!) I have not had problems with leaks in the radiators, water pumps or head gaskets. I do keep the heads torqued to 55#, use a leakless pump built by James Rupert and the sealed radiators were built by Brassworks. I often drive an entire season without the need to add water. This car will run the freeways at 65 mph in overdrive all day long no matter the hills or the temps if pressed to. (Not fun though!)
The second system recently installed and being tested with a Roof overhead valve head and an air conditioner is not working so well. The system is not leaking but it does not have enough capacity to remove all the extra heat from the souped up engine and the extra needs of the AC. So I need a bigger radiator but it will still be a sealed system. I expect it will also work flawlessly when sized correctly. If I tried to cool this with an open system, it just would not work.
From my experience, if the sealed systems are installed correctly and all the components are in good condition, they work without issue. I have also installed six additional sealed 4# systems on customer's cars who needed more capacity than the stock system could provide. These people like to drive fast and a stock system would not provide enough cooling.
Now that said, I have a stock open system in my '28 pickup that I drive 500 miles or less a year and not much over 55 mph. This open stock system works too! I have worked on many customer's stock cooling systems and find when everything is well maintained there are not issues.

If you drive your Model A like it was designed to be driven, a well maintained stock open system is all you need. If you drive faster, load your motor with non-stock systems or don't want to add water all the time you may need the extra capacity a pressurized system provides.

I believe if one is to fairly and wisely comment on the use of a pressurized system in a Model A, one should have enough experience with both stock systems and pressurized systems to know when a sealed and slightly pressurized system is the right choice. I admit I am a bit sensitive to the negative comments I hear and read about sealed systems. I often feel the need to reply to the "That's just stupid" or similar comments I get when I have my hood up. I try to be polite but it is getting more difficult as the source of the comment often ignores the fact that I am standing right there. I wasn't raised to do that or to take that! So, am I biased and sensitive? Yes!
I often respond to that type of comment by telling a little story and it goes something like this: I am usually close to the last car in the line of cars when we participate in Club tours and I always stop to help those having trouble. To help all the touring people, I carry lots of spare parts and two jugs of water in my trunk and use the water occasionally when there is a boil-over but never in my own car! Enough said?
Just my 3 cents worth on the subject!

Good Day Everyone!