Where I work, we have about 3700 pieces of 5304 telephones, so replacing them with new model is a capital investment, currently not feasible. I've faced the issue described in this thread, sometimes on brand new phones if they were sitting inside the box, long enough.
My finding is that the hook spring is insufficient to do the task. It compresses over time, and that causes the poor plunger contact, which can easily be interrupted by pressing buttons on the keypad, especially button 1.
There are other symptoms as well, depending on the condition of the spring.
So I took it upon myself to do some basic calculations, and based on my measurement, I picked a spring in McMaster-Carr catalogue. Spring is a bit different, but it's close enough, and unlike the Mitel spring which has no coils, MCM-C spring has 3 of them. Once I installed them, my measurements were confirmed correct. 4 years now, the telephones with new springs are still working.
The spring that I got is catalogue number 9271K31, if anyone wonders.
Name: Torsion Spring
Type: 90 Degree, Left-Hand Wound
OD: 0.315"
For Shaft Dia.: 0.187"
Wire Dia.: 0.035"
Leg Lg.: 1.25"
Number of Coils: 3.25
Max. Torque, in.-lbs.: 1.071
Material: Music-Wire Steel
Pkg. Qty.: 6
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