Rear brake wheel cylinder

Have you tried adjusting the rear brakes?  I've had similar symptoms and it usually means that the rear brakes, which require periodic adjustment, have worn.  You also could have leaking O-rings or gaskets in the rear axle, allowing axle grease to contaminate the brake shoes.

The brake drum and pads have less than 800 miles on them just rebuilt. But we dud not change the brake cylinders. 

The front disks, just from their design, are expected to have the pads very lightly touching the rotor at all times, even when the pedal is neutral.  That means you have zero-10 thousandths of an inch, max, between the rotor and pad with pedal neutral.

The rear drums, OTOH, with pedal neutral will be 20 to 100 thousandths away from the shoes -WAY more than the front disks.  When you step on the pedal, even though (and maybe, especially though) you have a 2-circuit (front/rear) master cylinder, the front pads hit the rotor way sooner than the rear shoes do, halting pedal travel, so the rear shoes don't make contact with the drums (or don't apply much pressure back there).

What'chagonnado?

Well, this is a common problem with a simple answer.  You can adjust the rear shoes in a lot closer to contact the drum with less pedal movement, but then the shoes will drag against the drum all the time and not work right - Two different brake designs entirely.  

So......The other thing you can try is running a "residual valve" in the brake line going to the rear brakes.  This is designed to hold pressure in the rear brake line to keep the shoes closer to the drum hydraulically and minimize pedal travel.  Since your master cylinder is at about the height of the rear wheel cylinders, you will need a ten pound residual valve and will have to rework the brake line going to the rear to accept the residual valve in the line.  The valve can be mounted anywhere in the line between the master cylinder and the "T" at the rear, to affect both rear brakes.  I put mine right beside the driver's seat, but with the carpet covering it, it can be anywhere along the floor.  It will never require service once installed, unless the line connections leak.  Adding a residual valve will greatly reduce the front wheel lockup, bring the rear brakes back into the story and stop you faster, overall.

Good luck!

Just saw the part about the rear cylinders.

There are several different rear cylinder bores available over the years as VW changed brake design for larger engines, but the IMPORTANT measurement is the size of the hole in the backing plate that the cylinder fits into (there are two sizes) and the centerline of the cylinder piston versus what your brake shoes expect (there are several different ones for different width brake shoes over the years).   If you get cylinders for the year of your car/pan you should be OK (don't worry about engine size for now).  It doesn't much matter if you run German or Brazilian cylinders, but from personal experience the German ones will last longer (many years) before failing and leaking than the Brazilian ones (years) and the Asian ones (months).  Either ones will stop you.  Your current problem is getting balanced front to back.

Noted on the 10 pound residual valve.  I will try that as a first step. Under nomarl braking everything is fine. Just if i hit the braks hard i lock up on the fronts and screech to a long amd sliw halt. 

Thanks gentlemen for your kind help 

The residual valve will help a lot, but remember that compared to a modern, front-engined car you have very little weight in the front.  Just because of that, you will still find that one or both front wheels will squeal a little when stopping on painted lines or hard braking around a 90º corner - it's just because the front is so light.   

With the valve in there, you should see a noticeable increase in stopping power.

Don't forget to get the rear shoes adjusted, too, just to make sure they're where they should be.  Find an old guy who remembers how to adjust VW beetle rear brakes.  There's nothing special about doing them, but not a lot of people remember how these days.  Bentleys or Chilton service manuals both outline how to do it.

Update:

Dual master cylinder installed (German) and 10lb residual valve installed.  The brakes feel good.  I tried slamming on the brakes and the front did not lock up.  But I guess there is nothing like real world stops.  I did not replace the wheel cylinders.

As for the reservoir I had a single tank that lived inside the trunk and I was able to replace it with a dual reservoir OEM that fit in exactly the same position as the two tanks hand the same profile only the dial tank was longer.  We needed to double the length of the strap.  Had a lot of fussing around and drilling a few extra holes and routing the line for the reservoir but it worked out.  Two hard line exit the tank connecting to two rubber hoses connecting to the MC.

The main reason for not installing the tank on top of the MC was that the tanks I found had much larger exit pipes that in no way would fit into the 7 or 8mm rubber grommets provided.  I that that it is for later style MC.

Also put some braided lines upfront.

Bill

Success!

(I always like a success...)

Here are some pictures of the reservoir. 

Here is a picture I found on the web that looks like my set up.