After some years of use of my 1989 MCAL, I still think it's a wonderful machine, capable to produce wonderful cups.
I found that the most annoying issue is the absence of an anti-vacuum valve.
This means that every time I use it, after about 10 minutes I have to manually open the steam wand to let the "false pressure" go out.
I know that newer units have an anti-vacuum valve installed on the top of the boiler.
Now I would like to add it on mine also.
I'm wondering if it could be possible to drill a hole on the top of the boiler to use the same anti vacuum valve as the newer models.
As I think it could be really difficult, I'm thinking also to add the valve to the steam wand pipe, putting it just before the steam wand valve.
In this case I would need to add a T connector before the steam wand valve.
Any suggestion?
alberto

Drilling and tapping in the boiler is very precise work and I would never advice it.
The boiler surface isnt flat and generally too thin to get enough thread in the hole to make it seal properly.
You really don't want to take the risk rendering your boiler useless if it's done wrong.

In some rare occasions there are these little platforms on the boiler where piping and fittings goes if the same boiler is used on different models of the same brand. I've seen it on Bezzera machines before, but I highly doubt this goes for the MCal

IMO the best solution is to find any fitting (preferably the boiler overpressure safety valve), use a T-fitting and add the vacbreaker to that. Be sure the vacbreaker gets mounted in a vertical position. If it sputters a lot of water before sealing, you can find one with a hose barb on it.

I am not sure how the brass tops of the older MCALS are made. On recent ones the brass on top seems to be quite thick.

Doing this would not require too much shop machinery and tools. I think the threads are 3/8" BSPT, but I am not certain. Taps this size are readily available. The hole certainly should not be drilled without a drill press to guarantee it to be absolutely straight and parallel to the boiler walls. The tap would need to be set properly too. (My practice is to disconnect the power to the drill or milling machine, put the tap in the chuck, and turn the chuck to get the threading started. You do not want off centre threads!!!)

It probably would not be a good idea to do this without substantial metal shop experience. You should be absolutely certain of what tap drill and tap is required too.

I must admit that I like the idea of installing the valve on the top of the boiler.
Installing it on the way to the steam wand would result in an uncomfortable shift to the right of the wand itself and the machine would probably loose part of its beauty with the valve installed in a visible place.
Actually another possibility is to install a T-connector under the pressure gauge, with the horizontal connection oriented towards the rear of the machine.
This way the pressure gauge would simply be a little higher, but the look of the machine would not suffer too much.
On the horizontal connection I would install a curve to make the pipe vertical again and then I would install the anti vacuum valve on top of it.
This way the valve would be not visible, being on the rear of the gauge.

It would be extremely easy to machine an adapter between the sight glass and the pressure gauge. I would do it by getting a piece of 25mm brass rod, cutting it to perhaps 25mm length, and then I would turn down one end to the diameter to permit using a die to thread the part so it would screw into the top of the sight glass. I would bore it on the other side and use a tap to thread it to accept the pressure gauge. I would mill off one side of the brass piece to make it flat and then drill it to accept the diameter of the vacuum relief valve. This could be done easily and make the part very unobtrusive.

One could also machine a vacuum relief valve into a piece of brass, prepared as described above.

If you wanted to drill the boiler it might be a good idea to get a piece of flat brass stock about the boiler's thickness and drill and tap that first for confidence to know you can do it without damaging the boiler.

Brass is extremely easy to machine, especially some brasses. I am almost sure that the standard MCAL vacuum relief valve is 3/8" BSPT. This is the size used by most portafilters. The standard tap drill size is a weird inch size, I think 37/64" or something like that. That is a bit bigger than 15mm. I have used that size to make portafilter spouts on more than one occasion.