1967 Mustang Brake Light Problem

Need some help with a Brake light problem on a 67. Power Assisted Drum Brakes all round as standard
At rest brake lights work fine A slight jab of the pedal and on they come BUT as soon as I start driving I need to jump hard on the brakes for them to work. Bloody dangerous as I can't "feather " the pedal to toggle the break lights.
Blinker lights work fine and seem do do what they should
With car running and in park after vacuum booster has done it's job the brake lights work with slightly more pressure than when ignition is off
Emergency Flashers, Blinkers and horn all work fine despite problem I had some 2 years ago or so with the horn as I outline below. I'm not sure if the brake problem presented straight after the horn problem or whether it existed prior or is more recent. Car not driven much and rarely receive feedback on whether brake lights are functioning. Started looking recently after being informed they where not functioning
History
I changed the original steering wheel to a Grant All seemed OK . Some months after changing Steering wheel backed car out of garage to let it run for a while as it had not seen the daylight for around 6 weeks.
Back in garage and noticed a ticking sound when car shutdown. Noticed tick coincided with interior light flickering on and off Quick disconnect of battery and found Grant horn button red hot. Revisited the insulation I had on the horn wiring and all seemed well Horn works etc. Some time later whilst on road informed I had no brake lights Tested at rest and all fine so assumed dirty switch. Problem however has continued _ Brakes test fine at rest but when travelling I have to slam them on to get the brake lights to work. I have changed the brake light switch and no change. Even added a 1/8" plate to the contact area of the switch and found at rest the brake lights stayed on so clearance or play doesn't seem to be the problem and hence removed the additional plate
Only other mod has been to add a small tacho
It's doing my head in - Work at rest but not when on the road travelling along ?????

2Blue2 said:

I'm assuming that it has the original wiring harness?
Brake Light Works at rest when grounds are not shared
Running, grounds are shared.
Old wires and dirty grounds / contacts and battery terminals
Make sure engine and battery are grounded to chassis, and clean all other points of contact
Intermittent problems are the hardest.
Good Luck and post your progress.

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Yes all original except Brake light switch now
Thanks all for the insight Still have the problem although I have not gone too far with the investigation other than give some wiring a wriggle !!
Question - If it was an earth problem and seeing many bits share earths when running if I was use a jumper lead to establish another earth on the tail light globe ( I assume the earth is the spade connector on the ring on the globe sticking out of the mounting cup ( Am I correct ??) would this do any damage and if not would it be a pointer to an earth problem??
Guess I am looking for an easy way to start before chasing all the earthing points , removing the steering wheel etc
The replacement brake switch I used was a Scott Drake with no indication of "pressure" It is doing exactly what the original was - Works fine at rest but when travelling have to stomp hard on the pedal for brake lights to work and even then they are sightly delayed in lighting up. Surely there is no correlation between auto transmission gear selection and the brake switch?? Is there??
As mentioned in the first post I placed a 1/8" shim on the switch contact plate which resulted in the brake lights being on permanently at rest Didn't try running VS brake light operation with the shim in place
Also I note that the proportioning valve which I assume would be standard and original to the car has an electrical connection coming out of it from a nylon fitting - can anyone explain it's purpose?? Where does it terminate? Would it affect brake light operation at travelling speeds?

zookeeper said:

Since the switch is what turns on and off the brake lights, did you even try adjusting it as per the link I posted? Might be worth a try before you go any farther...

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Yes I now have attempted to adjust without any change to my problem I have also been placing various thickness shims on the rod /switch contact surface to minimise the travel distance but problem still exists. I even placed a shim which would have been just under an 1/8" between the contact surfaces and this resulted in the brake lights being on at rest. Run the car at about 5 mph like this and thought if the problem was some sort of short the lights may turn off BUT no they stayed on.
I am at a loss.
What would you think if I disconnected the blinker circuit at the globes and tried the brakes again Might it point to a problem in the turn/brake switching? Would it cause any wiring damage if I made sure any bare ends where well insulated before adding any power?
Could the problem be voltage / regulator related with possibly higher volts with the engine under load causing the problem??

Thanks Zookeeper. I have tried bending the tabs , probably up tp 1/8" forward without success atm . I have cleaned the battery lead at the block end , and had a quick look over the wiring, even disconnected the taco I installed and still no joy. Seems when at rest less pedal travel and I can get the brake lights to toggle. On starting the car and the vacuum booster doing it's thing I have to move the pedal further to toggle the brake lights yet when I start moving NO brake lights until approx 1 second after the car comes to rest. I ran the car at dusk with the lights on ( using wife as a spotter - yeah I know first mistake !!) and still no change to brake light operation ie OK at rest and not coming on till car has stopped after rolling down the road. Wife reckons no change in brightness of lights when they did come on with headlights turned on , although I do notice a change when stopped in the garage with and without engine running. I will continue bending the tabs until I reach the point of lights permanently on the back off a bit to the not on spot. Question - Is the flat on the rod long enough to meet the tabs before the centre of the switch? ie will the rod flat miss the tabs even if they are bent? Went to get a relative measure last time under but couldn't quite get it accurately that time
Anything else anyone could suggest I check?
What about disconnecting the blinkers to see if the problem lies in the cut out switch between blinkers and brake lights, although it works perfectly at rest
Thanks in advance