Originally Posted by
mark a.
"When the rear cylinder fails, it will be a catastrophic total loss of braking" Not true. There is a safety valve in the prop block that prevents this. When it detects a leak it will shut off the system that failed, fronts or rears. How can you say with any degree of certainty his master cylinder is bad without ever even checking it out ? I only replace parts that are bad. It's bad business to sell a customer calipers, rotors, etc. that he or she doesn't need. Fear mongering & blindly throwing someone elses money at this problem is no way to fix this problem.
If you can't get anymore air out of them, I would look into the proportioning valve's safety switch.
No competent mechanic would permit only one side of a caliper to be repaired / replaced without doing the other one --- at the minimum, a complete rebuild / retest.
There is no evidence that the OP even did what would be minimally required in a lawful brake pad replacement job:
Namely, checking rotor thickness, and if safe, resurfacing it before placing new pads / calipers in.
When only one side of each axle is "repaired" in this manner, there is a risk that the other side (of unknown condition) will fail shortly thereafter.
What is at risk is the other side will shortly thereafter, either fail or jam --- resulting in uneven braking that can cause the vehicle to veer to one side.
For exactly the same reasons, tires are matched on the same axle, and no competent mechanic will tell you "one side of the brake good" and just replace the worn pads on the other side, or allow you to mix and match different types / construction of tires on the same axle.
As for this:
There is a safety valve in the prop block that prevents this. When it detects a leak it will shut off the system that failed, fronts or rears.
The proportioning valve is not intended to deal with front brake failure.
It only "proportions" the rear, not the front brake action.
It is not known if this vehicle is ABS equipped, and if so, whether the proportioning function has been replaced with the ABS programming.
StopTech : Balanced Brake Upgrades
If it is in fact the front brake portion of the master cylinder that have failed (piston seal failure is most common), then the leaked fluid is ending up in the reservoir, perhaps with some going into the rear master cylinder if the seal there has failed too.
Without a disassembly and bench testing, there is no telling.
However, if the brake warning light is "on", most likely, it is the sensor detecting either a low brake fluid condition, or the pressure differential between the front and rear brake systems as being excessive.
Are you trying to tell the OP that it is lawful and safe to continue to operate a vehicle with the brake warning light on?
Sure, try a bleed... but if that doesn't instantly fix it, it is time to see a qualified mechanic.
OP do not appear to be a licensed mechanic in any jurisdiction.