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MG MGB Technical - brake fluid
|First, happy new year to you all.|
Now that i have to replace rear wheel cylinders( fail MOT), what is the best way to change the brake fluid.
its a MGB 71 without brake booster.
Flush the system out with denatured alcohol before you put the new brake fluid in. You should be able to get denatured alcohol at any good paint supply store. Pour new brake fluid into the system through the master cylinder. Open a bleeder screw, then use the brake pedal to pump the brake fluid through until it comes out the bleeder screw, then close the bleeder screw. Repeat this process on all four wheels, then bleed the system.
Brake bleeding has always been an inconvenient and messy job. Commercial shops typically use an expensive pressure bleeder to swiftly do the job. Unless used properly, a pressure bleeder can introduce air bubbles into the system through cavitation. Vacuum bleeders are less expensive but can also introduce air into the braking system. The time-honored manual method (still universally used in racing) requires two people: one to pump the pedal, and another to open and close each bleed screw in sequence. Proper procedure and coordination are required in order to prevent air inclusion. A Solo-Bleed bleeder screw (Part # 280022ERL) solves both problems. This device makes use of a spring-loaded plunger anti-backflow valve in order to allow one person to safely bleed brakes without a mess and with no chance of air inclusion. It is a direct replacement for the original equipment bleeders. This handy item is marketed by Holley Performance Products and can be found at their website http://www.holley.com/search.asp
|I just went through this on my '71. My intent was to bleed the rears only using the 'pump the brakes' method. This doesn't work for the simple reason that the fronts are still actuating and preventing the brake pedal from obtaining sufficient travel to force any meaningful amount of fluid to the rears. Sooo, I eventually came to the conclusion of releasing (de-bleeding) the fronts so I could bleed the rears. Then follow up with the fronts, which works just fine.|
The other two methods (other than the 'pump the brakes' method, which requires a partner), are to force the fluid via pressurizing the master cylinder through the lines, or to suck the fluid out from the brake cylinder end, while keeping the master cylinder topped off of course. I don't own the equipment to suck the fluid, but do own the easy bleed equipment for pressurizing the master cylinder. Sadly, I've had little luck with this method, as I can't seem to obtain a decent seal at the master cylinder and fluid goes everywhere. So bottom line, since I have a partner, I stick with the 'pump the brakes' method to bleed the system and it works pretty well, as long as the master and wheel cylinders are in good condition. I've heard others use a fourth method, of opening up the bleeds at the wheel cylinders, and letting fluid gravity feed over time from the master cylinder, but I've had little luck with that, perhaps due to lack of patience.
Best of luck...shouldn't be too hard.
|Single circuit or dual?|
Probably the eeziest (ho ho) way is to use an EeziBleed to pump clean fluid through. On a single circuit system one bleeds starting with the longest run (left-hand rear) and progressively does shorter runs finishing at the right-hand front.
With dual circuit you have to do things with the pressure failure warning switch. One set of instructions says to remove the wires from this and unscrew it 3 1/2 turns, bleed at normal, then tighten the switch and reconnect the wires. That may be for the switch in the remote manifold and unboosted brakes.
FWIW I think on the later dual master with integral booster and failure switch, you bleed as normal first one circuit then the other. Then with the ignition on the failure light should be on, so bleed the *first* circuit again very slowly until the light just goes out.
|Paul Hunt 2|
This thread was discussed on 05/01/2008
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