On my Pioneer, O does not activate until bonus is zero. Once bonus is zero, the motor turns 120 degrees and the O relay pulses, firing the ball return kicker once.

eg. With the playfield up, I start a game and load up 1300 bonus.

I tap the outhole switch. Motor turns 120, bonus steps down 5 times.
I tap the outhole switch again. Motor turns 120, bonus steps down 5 times.
I tap the outhole switch again. Motor turns 120, bonus steps down 3 times.
I tap the outhole switch again. Motor turns 120, O pulses and the ball return kicker fires.

If I tap the outhole switch again, the last step repeats.

I'll use the word energise for when the relay turns on/pulls in and de-energise for when it turns off/lets go.
I'll use the word fire for when a coil energises and then de-energises quickly.

With 1300 bonus, when the ball drains:

The motor turns 120, during which the Subtract "Bonus" Unit coil fires 5 times.
The motor turns 120, during which the Subtract "Bonus" Unit coil fires 5 times.
The motor turns 120, during which the Subtract "Bonus" Unit coil fires 3 times.
The motor turns 120, at the end of the turn, the O relay energises, which causes the Ball Return coil to fire, kicking the ball, and then the O relay de-energises.

Looking at the schematic, the O relay should NEVER energise until the bonus unit has reached the zero position:

Make break switch circled

If I get a chance this afternoon, I'll fiddle with that switch and see if I can replicate your problem.

Ok, I had a chance to have a quick play around and see what I could come up with. I managed to get my machine to kick the ball back after only 5000 of the bonus has been scored.

eg. Game with 1300 bonus, ball drains, bonus counts down 5000, ball kicks out, leaving 8000 bonus still lit

All I needed to do was short the upper leaves on the make/break switch on the bonus unit, the white blue and white orange. Shown here open as the bonus unit is not in the zero position: 20170723_193455 (resized).jpg

The first diagnostic I'd do is checking that that switch really, really is open when then bonus unit isn't zero - I'd put a piece of card between the upper contacts or de-solder the white orange from the top tab.

This should make the machine count down all the bonus but no longer kick the ball out. If that's what happens, there's something wrong with how that switch is working that is isn't opening properly.

If the problem continues, there's a short between the white orange and white blue somewhere else.

Ok, the O relay pulling in 5 times is definitely not right. As the Kick Out coil doesn't fire five times, I'd guess the make/break switch on the bonus unit is actually ok.

The motor sequence chart only has one stack that fires 5 times - 1A:
Pioneer Motor Sequence Chart

So I think something is going on with 1A, eg a short between 1A and something that is causing O to pulse 5 times.

That's really good news.

Oh excellent! I thought I hadn't quite replicated what you were seeing, but it maybe there was a bit of a language block/misunderstanding.

You are most welcome. It was an interesting exercise looking at the schematic and trying to picture what could make the machine behave the way you were seeing.