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Ron Kwas · Dec 1st, 2015, 01:10

Dave;

Welcome to this Forum.

Poor Hydraulic Brake Light Switch function issue can be explained by the interaction of three separate factors:
1. Common practice of sensing hydraulic brakeline pressure (which was typical for many manufacturers in its day) This means that rate of pressure rise is slow (that's because we are taught to decelerate and apply the brakes slowly...and that also means the contact will close slowly (see: 2), if we jab on the brakepedal while driving (panic-stop!), or in testing in the driveway when vehicle isn't moving, they will typically work fine, but in a real-world driving situation if we were to apply pedal pressure like that, (see: 3) we would put our passengers into the seatbelts, and few people will want to ride with us.)
2. Switch Design Construction. All failed switches I've ever inspected or performed a post-mortem on, the closing contact was essentially under direct control of the pressure and without any fast-make-fast-break mechanism in sight. This is common practice in electrical industry switches and it serves to slam the contacts closed (typically accompanied by a click or snap of the mechanism), preventing arcing/carbonization during slow closing, and cutting through what little arcing and carbonization does occur, to assure contact. In Hydraulic Brake Light Switches, such a mechanism, which would accommodate the slow pressure rise, is conspicuously absent. I doubt any manufacturer will want to incur costs now, of redesigning this ancient technology for the market of selling a limited number of replacement...little cost/benefit!
3. The Driver Factor. People who apply their brakes with a quick jab, then back off and apply gently rising pressure will experience switch failure less. I "Brake like an Animal" when I'm alone, and would cause even a hydraulic switch to function pretty well, but if I'm taking my grandma to buy cheese danish, I doubt my hydraulically switched Brake Light were working so good.

The motor vehicle industry also saw the light, and unanimously went away from Hydraulic Pressure Sensing Switches (HPSS) and to Pedal Position Sensing Switches (PPSS) in the late sixties, in part recognizing the weaknesses (slow activation of Brake Lights being only one of them!), and for all the benefits. I expect that maybe even Mini and Minors made after that time period were equipped with this style...my advice would be to upgrade ALL earlier vehicles with these PPSS!...don't mess with the HPSS, and don't expect manufacturers to design a better replacement switch...it's kindof like asking them to design a leather strap Friction Brake...we're waaaayyy past that!

If you are referring to this "detailed article" (http://www.sw-em.com/hydraulic%20bra...es%20notes.htm), you are welcome to reference it to whomever you like if you give credit to the source and author.

Cheers from Connecticut!

Dec 1st, 2015, 01:10	#9
Ron Kwas Premier Member Last Online: Today 11:01 Join Date: Jul 2007 Location: Connecticut, USA	Dave; Welcome to this Forum. Poor Hydraulic Brake Light Switch function issue can be explained by the interaction of three separate factors: 1. Common practice of sensing hydraulic brakeline pressure (which was typical for many manufacturers in its day) This means that rate of pressure rise is slow (that's because we are taught to decelerate and apply the brakes slowly...and that also means the contact will close slowly (see: 2), if we jab on the brakepedal while driving (panic-stop!), or in testing in the driveway when vehicle isn't moving, they will typically work fine, but in a real-world driving situation if we were to apply pedal pressure like that, (see: 3) we would put our passengers into the seatbelts, and few people will want to ride with us.) 2. Switch Design Construction. All failed switches I've ever inspected or performed a post-mortem on, the closing contact was essentially under direct control of the pressure and without any fast-make-fast-break mechanism in sight. This is common practice in electrical industry switches and it serves to slam the contacts closed (typically accompanied by a click or snap of the mechanism), preventing arcing/carbonization during slow closing, and cutting through what little arcing and carbonization does occur, to assure contact. In Hydraulic Brake Light Switches, such a mechanism, which would accommodate the slow pressure rise, is conspicuously absent. I doubt any manufacturer will want to incur costs now, of redesigning this ancient technology for the market of selling a limited number of replacement...little cost/benefit! 3. The Driver Factor. People who apply their brakes with a quick jab, then back off and apply gently rising pressure will experience switch failure less. I "Brake like an Animal" when I'm alone, and would cause even a hydraulic switch to function pretty well, but if I'm taking my grandma to buy cheese danish, I doubt my hydraulically switched Brake Light were working so good. The motor vehicle industry also saw the light, and unanimously went away from Hydraulic Pressure Sensing Switches (HPSS) and to Pedal Position Sensing Switches (PPSS) in the late sixties, in part recognizing the weaknesses (slow activation of Brake Lights being only one of them!), and for all the benefits. I expect that maybe even Mini and Minors made after that time period were equipped with this style...my advice would be to upgrade ALL earlier vehicles with these PPSS!...don't mess with the HPSS, and don't expect manufacturers to design a better replacement switch...it's kindof like asking them to design a leather strap Friction Brake...we're waaaayyy past that! If you are referring to this "detailed article" (http://www.sw-em.com/hydraulic%20bra...es%20notes.htm), you are welcome to reference it to whomever you like if you give credit to the source and author. Cheers from Connecticut!