'63 R1 Hydraulic brake switch

[DobbM]

Still waiting for my shop manual to arrive.

Thank you for any advice.

It takes the brake pedal to be pushed about 95% of full range to activate the brake lights. Aside from slamming on the brakes for an unexpected obstacle, I don't think I ever really have the pedal go that far. It appears to be a hydraulic switch on the master cylinder, so not adjustable. Correct? The fluid is full, and since a trusted mechanic replaced all the brake lines with stainless, I'm sure that they were bleed properly. Also, the brake booster was rebuilt, and a new master cylinder installed. The brakes themselves work fine, and no squishiness, etc.

A). Should I try and bleed the lines first to help diagnose (or if I need to replace the switch, I'll have to bleed them again).

B). Do the switches fail? Or should I be looking elsewhere for my perceived issue?

C). If they do fail, where might I start hunting for a new one?

[Gunslinger]

You have "DOT 5" on your master cylinder cap so I assume that's what's in it.  Silicone brake fluid is notoriously difficult to bleed all the air out of...the little air bubbles like to hide wherever they can and defy being bled out.  If the car had standard DOT 3/4 fluid in it and wasn't completely flushed out and all the hoses replaced you will have spongy brakes or even fail in rare cases.  The best performance cars don't use DOT 5 as strong cornering can cause bubbles in the system.

DOT 5 also can kill the hydraulic brake switches...why I don't remember but it was explained to me once.  

The use of DOT 5 might be the cause of your problems.  

Edited yesterday at 02:33 AM by Gunslinger

[mfg]

Guslinger made some good points… Also, those hydraulic switches commonly fail… When in doubt… change it out!!!

Or convert to a mechanical brake light switch.

[Gunslinger]

The hydraulic brake switches suffer from a dual problem beyond aversion to silicone fluid…current ones are made offshore now and simply don’t last…and original, well made switches are old enough now that they’re suspect in how well they work for any length of time.  

[DobbM]

@Gunslinger thanks for that tidbit of info. Yes the entire braking system was replaced and it uses DOT 5. I did a bit of research, well, I had AI search for me and compile an answer from sources. I'll

Have to next research 1). How to convert to a mechanical switch, and until then, 2). Find a replacement hydraulic switch.

1. Corrosion and Electrical Arcing (The Most Likely Cause)

The primary theory, which is backed by inspection of failed switches, involves the non-hygroscopic nature of DOT 5 fluid interacting with the internal electrical contacts:

Water Pooling: DOT 5 (silicone-based) does not absorb water (it is non-hygroscopic), unlike DOT 3 or DOT 4 (glycol-based). Any moisture that enters your system (which is unavoidable, even with a sealed system) remains as a separate droplet. This water tends to sink to the lowest point, which can be the caliper, or, critically, the sensitive chamber of the brake light switch.

Corrosion: This pooling of water causes localized corrosion on the internal metal contacts of the switch.

The Switch Design Flaw: The vintage hydraulic switches (like the one on your Avanti) are not "snap-action." The contacts close slowly as pressure builds. This slow closing action causes arcing across the contacts. In the presence of the silicone fluid and the localized moisture/corrosion, this arcing creates a thin layer of non-conductive carbonization or corrosion that prevents the circuit from closing reliably. This is why you have to press harder and farther—you need more pressure to physically push through the layer of crud on the contacts.

2. Viscosity and Compressibility

Soft Pedal: Silicone fluid is slightly more compressible than glycol fluids (DOT 3/4). While negligible in hard braking, it can create a slightly spongier pedal feel.

Activation Threshold: Due to the soft feeling, some enthusiasts report that it takes a higher initial line pressure to overcome the switch's internal spring/diaphragm to activate the light, causing the exact symptom you are experiencing.

What to Do About It

If your Avanti currently has DOT 5 fluid, you are faced with a choice:

Replace the Switch Seasonally: Many classic car owners running DOT 5 are aware of this issue and simply replace the cheap hydraulic brake light switch every year or two as part of their maintenance schedule.

Convert to Mechanical Switch: The most reliable solution, which many Avanti owners resort to, is to remove the hydraulic switch entirely (and plug the master cylinder port) and install a modern, adjustable mechanical brake light switch on the brake pedal arm under the dash. This is the most popular permanent fix for this decades-old issue.

Full Fluid Conversion: Converting back to DOT 3 or DOT 4 is a massive undertaking. The silicone fluid permeates the rubber components, and mixing the two types causes a gummy sludge. To switch back, you would need to completely replace all rubber components (hoses, seals, wheel cylinder cups) and meticulously flush all metal lines, which is often prohibitively expensive and difficult.