Another wizardry: 2 buttons for the same purpose?

[dorchester]

Here is what I've got in my car:

drive.google.com/file/d/1Snm9I5Zk4Z383MFGwafAaHjxn4QKPpQO/view?usp=sharing

drive.google.com/file/d/1R6w0t4vx4ICqE7KHjX0KT32BP1jnk2Dx/view?usp=sharing

But regarding the toggle switch on the dash, the booklet says it's for the heater! I'm lost...

Yesterday I went out of petrol: I didn't imagine the garage spent so much fuel doing the tests about carbs, idle & advance and - fool I was - I relied upon the accuracy of the gauge! I intended to refill the tank but I thought that 1/4 was a 1/4! Actually it's around 1/10!!!

I tried both solutions (dash & under dash) but no way to gain access to the reserve and it looks that going uphill - that was my chance! - the mechanical pump was loosing its capacity and that I should have reacted at once.
BTW my seller told me that never I shall use the petrol reserve control under the shelf so that he'd put a red tape on it. So he had been told.

I assume after a close look on the forum exchanges that it's very risky to go above the 1/4 mark.
But anyway who could explain me:

1. why I've got 2 similar functions and why it's not mentioned in the booklet? Was there a trick or change operated by a previous owner?
2. how to do the trick and save oneself switching on the reserve if it may happen once more (but I'll try not to do the same mistake twice)?
3. In case the reserve should never be used, does it mean that I've got in fact around 57 litres usable and never more?

Thank you to help me resolving that dilemma.

N.B. the last time I went out of petrol it was in Greece in 1976... Not so happy days!

[percy]

The switch on the binnacle is not original to your P5B and likely comes from a P5 with twin SU electric pumps. That place on the dash is normally occupied by the heater switch.
I assume you have a mechanical fuel pump in your car?
The correct control is the lever marked fuel reserve. When it is pulled, it operates a simple tap which then opens up a reserve supply from the tank. In practice, this is simply an extension to the main pick up so sits lower in the tank, hence a reserve supply. It's a simple and effective system but often the tap is seized through lack of use or will leak after a period of no use, the pull cable is rusted and often breaks, the reserve hose is blocked etc. The solution is an overhaul. The brass tap is very simple and usually just requires a new VITON O ring.

WARNING: If anyone works on the system, it's essential to disconnect both feed hoses from the fuel tank BEFORE removing the tap otherwise fuel will syphon out of the tank. 

[dorchester]

Thanks a lot Percy for your enlightening and comprehensive answer.
At least I've just solved half of the mystery (I should have done it before asking...): actually the binnacle heater blower is working fine on both positions fast on the right and slow on the left, but I wonder why a prankster stuck on such a misleading label!

Hence it's the remaining bit, the fuel reserve handle below the shelf which looks rather stiff. I guess something may be seized. I hope nothing's broken but I'll ask the mechanic to have a close look next Wednesday and I'll transmit him your warning.

EDIT: yes I've the genuine mechanical fuel pump which is likely working well. The evidence is the last appalling mpg...

[dorchester]

Important question before testing:

If I pull the 'fuel reserve' under the shelf while my tank is nearly full what may happen?
Can I actually be using the reserve?
In case the system is stuck or blocked even if I can pull the handle, is there any risk to run out of petrol and defuse the mechanical pump?
Thanks a lot.

[percy]

There are two outlets on the top of the fuel tank. One is the main the other reserve. They both lead to one single pick up pipe in the tank but it's designed so that the reserve pipe lies lower in the tank. That's the reserve. If you pull the reserve lever, the fuel will be picked up until it falls below the end of the reserve pipe then you really will have run out of fuel.

If the system has been neglected or not used for years, the following scenarios are possible:

1) The lever will come out in your hand followed by a meter of rusty wire.

2) The reserve tap will leak.

3) No petrol will flow because the fuel pipes are bunged up or the reserve tap has stuck in a half-open position.  

4) The tap will not return to its normal position when you push the handle back in due to lack of grease in the cable and/or a broken return spring/stiff tap.

[dorchester]

Hello Percy. Thank you for your very clear & useful comment.
1. AFAIAA I've pulled and pushed a few times the lever and even a bit stiff to push it looks still in place and not broken at all
2. No smell and no visible stain inside or outside (I haven't taken off the boot protection but nothing on the ground nor under the car)
3. 4. I can't tell whether it's partially stuck or not but I've pushed back in place the lever a few times as well and done 11 mls without any problem and I've to do 11 mls more to reach the garage tomorrow morning.
Fingers crossed...