Bleeding front brakes only?

Hi, I have a 1989 ford escort mk4 and the brakes were OK but not very good. I knew that some air had gotten into the system before, via a leaking servo gasket (This has now been sealed). Yesterday I decided to bleed the brake system of air, and I did the usual of attempting to start on the rears (Drum Brakes), furthest away from the master cylinder at front. However, I found that both drum brakes had seized bleed nipples, so unfortunately I could not get them off. I then went and did the fronts (Disc Brakes) and the nipples where able to be undone. I bled these two (Again, furthest away from cylinder first) and the fluid is now shooting out of the nipple with a good flow, before it was only drizzling out.

Now the problem is I can pump the brake and it will start to somewhat regain a good pressure, then when I stop pumping and wait a few seconds… I then go to push the brake pedal again and it has lost almost all of it. There's only a little pressure.

Do you reckon I should either try and get the rear brake nipples undone? To bleed them, or if not I'll have to purchase a new set of rear drum brakes I guess. Unless it's also possible to remove the brake line to the rears that's holding the fluid… And bleed the lines or something?

Thanks for any reply's.

Added (1). I've tried doing it with the car off and the car running, doesn't seem to make a difference.

Soak bleed screws on rear wheel cylinders with good grade of penetrating oil for a couple of hours. Use the correct size 6 point socket or wrench, probably 8MM. Tap LIGHTLY w/ small hammer. Cracking the brake line to W/C won't bleed it.Worst case, buy & install new wheel cylinders, they're cheap. Bleed brakes as you described. STILL mushy? Bleed or replace master cylinder. By the way, just WHAT "servo gasket" is in the brake system?

Get the nipples open - soak with penetrating oil for a day.
I was taught to bleed the short pipes first see

You will never get it done without bleeding the rears as well. There's no point in attempting it.

You have to get the rears sorted.

BTW, you bleed them with the engine and parking brake off.

Hi so as it is so old it sounds like this now 25 year old car is trying to tell you. That you have had the best out of it time to scrap it.

Never bleed brakes while engine is running. You can undo the rear brake pipe conected to the wheel cylinder if you can't undo the bleed valves. Once the brale pipe is undone, but not fully, pump the brake pedal 10 times and use a jack handle bar to keep the pedal pressed down while you tighten up the brake pipe.