[?]
Rispondi CitandoDunque, continuando le ricerche sul web, forse ho svelato l'arcano...le valvole proporzionali sono state montate sulle Vette in epoca successiva alla mia, per cui è un semplice "interruttore", c'è una valvola che aziona l'interruttore in caso di forte differenza di pressione tra il circuito anteriore e quello posteriore. L'interruttore (in effetti è chiamato così anche sul service manual) accende la spia brake sul cruscotto (che si accende anche con il freno a mano inserito).
Questo "interruttore" quindi non regola in nessun modo le pressioni tra anteriore e posteriore come una normale valvola proporzionale...
A sentire poi sui forum americani fa + danni che altro, poichè non sono l'unico ad avere questi problemi..diverse persone infatti con questo interruttore (non solo sulle vette ma anche sulle mustang ad esempio) hanno difficoltà a spurgare e poca pressione al posteriore...
C'è chi ha elimato questo interruttore trovando benefici immediati, a proposito di questo vi segnalo per chi ha voglia di approfondire la relativa discussione :
http://wiki.corvettefaq.com/index.php?title=Brakes
Sono la 3 ultima e 4 ultima nell'elenco..."remov ing brake proportioning valve"
sono dei file zip che contengono le discussioni da cui è tratto :
People get confused about C3 brakes because we assume since they all have 4 piston calipers, they are all alike; one of the chief differences is whether or not your car has a combination valve or a proportioning valve. Here is a quote from the Haynes manual:
"Two basic variations of this valve have been used. On 1968 through 1974 models, the valve includes a pressure failure warning switch which illuminates a warning light if hydraulic pressure drops in either the front or rear brake circuit. On 1975 and later models, the valve also includes a porportioning function. Under heavy braking, the proportioning section reduces hydraulic pressure to the rear brakes to prevent early rear wheel lock-up. The valve is also designed to assure full pressure to one brake system should the other system fail. The design of the switch and valve are such that the switch will stay in the 'warning' position once a failure has occurred. The only way to turn the light off is to repair the cause of the failure and apply very high brake pedal pressure."
Also, I know it is conventional wisdom on this forum that brakes should be bled from the caliper farthest from the master cylinder to the nearest. This is usually postulated in threads where all the participants bemoan the difficulty of getting all the air out of the lines, or of achieving the much desired "firm pedal." This is a quote from an original 1973 Chevrolet Service Manual, copyright 1972 General Motors Corporation. Section 5-18 in the Brakes chapter.
"It is advisable to bleed one valve at a time to avoid allowing fluid level in reservoir to become dangerously low. The correct sequence is to bleed the valve, either front or rear system, nearest master cylinder first. This sequence expels air from lines and calipers or wheel cylinders nearest to the master cylinder first and eliminates the possibility that air in a line close to the master cylinder may enter a line farther away after it has been bled."
Now that is mere common sense. When you press the brake pedal to bleed the brakes, the fluid obviously moves toward the wheels, not toward the master cylinder. It also makes no difference whether you bleed the back or front wheels first. Note the two reservoirs of fluid in the master cylinder; they are separate and do not exchange fluid or air so long as the master cylinder is functioning properly.
Now, here's the kicker I don't know the answer to, since my 1973 Corvette has a combination, not a proportioning, valve. Many of you guys who have 1975 and later cars WITH proportioning valves should check into whether it needs to be mechanically held open during brake bleeding. During 1973, some other Chevy cars, not the Corvette, used proportioning valves. If you used pressure bleeding, the following paragraph applied to those cars:
"NOTE: On disc brake cars a combination metering, proportioning and failure warning switch is in the hydraulic system and mounted either on the frame rail or under the master cylinder. This valve must be held in the open position while bleeding. This can be accomplished by installing Tool J-23709 with the open slot undr the mounting bolt and pushing in on the pin in the end of the valve (fig. 38). Be sure to re-torque the mounting bolt after removing ToolJ-23709."
