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MG MGB Technical - to get brake fluid flowing in new wheel cylinders

hi again

79mgb

recently installed two new wheel cylinders.
(you all helped me with the circlip installation)

now that all is in place, i can't get the brake fluid to flow down from the reservoir to the cylinders.

what is the trick/secret? do i need some special tool, which i don't have, or what?
again i'm frustrated !

john
john sutter

That's strange John. With all in position it is just a case of opening the bleed valve and having someone push the brake pedal down, get them to stay at the bottom of the down stroke, tighten the bleed valve closed, then they can release the pedal and repeat the process, ie open valve, push pedal down, hold,close valve, release pedal. There may be a lot of air to bleed off initially, the Easy Bleed system works well in my hands (Guysons.)Mike
J.M. Doust

John,
master cyl/servo overhaul....
The open, down, hold, close system Mike suggests should work but the real problem is probably the none return valve(s) in the master cyl/servo.

Take the opportunity to renew all the rubber up front, renew all the fluid and save yourself time/trouble in the future.

MGmike

M McAndrew

John We never got the later B here in OZ but I understand the 79 had a split system and the valve that blocks a failed system has probably sensed failure of the rear brakes as you try to bleed them. There should be a valve you can loosen at or close to the master cyl to disable this while bleeding It is, on most cars between the brake pipes on the same unit that houses the failure warning switch. Denis
Denis4

Dual circuit will be a problem as pressure in the front circuit will limit the movement of the pedal and hence volume of fluid pushed though with each pump. Continuous flow with something like an EeziBleed (gunsons) is easier. You are supposed to unscrew the balance switch from the master by 3 1/2 turns before bleeding on the master with integral servo, which is different to the earlier unboosted master. The later imbalance valve does move across and lock if the switch is screwed in, unscrewing the valve now *should* enable it to move back to the middle. However I can't really see it blocking the outlet of the master when it *does* move across, because switch screwed in or not it is still going to move across with *pedal* pressure, which is going to prevent bleeding anyway. With an EeziBleed on low pressure I wouldn't be surprised if it didn't move the valve anyway.

Whilst the earlier dual master did have residual pressure valves I don't think the later (or the single for that matter) did, and they *shouldn't* prevent fluid flowing from the master to the cylinders anyway, unless they are faulty or blocked, and the latter would cause problems anywhere in the circuit.
PaulH Solihull


hi
79mgb

your suggestion is what we were doing. my son was pushing the brake pedal and i opened the drain to let out the fluid, close, then repeat----

we were doing this for many 'pushes" but little if any came out. we will try some more after the rain.

any more suggestions greatly appreciated.

john
john sutter

John, I hate to tell you this but you are going to have to trace back and see how far you are getting brake fluid in the lines.

I would first loosen the fitting at the master cylinder and be sure fluid was coming out there. For the rear brakes it is the union farthest toward the front of the master cylinder .

If you get a good squirt there, tighten it back up. If not you have a MC problem of some sort.

You might want to start at the T fitting on the rear axle where the brake hose attaches. Loosen one line there and see if you are getting fluid.

At some point you may find you have a clogged or a crimped brake line or even brake hose. Brake lines on the rear axle seem to have a propensity to get dented when the car is towed.

On a car I was working on recently I found that the brake hose was clogged and both brake lines on the rear axle had a dent on them which stopped fluid from reaching the rear wheel cylinders.

John in Music City
John Lifsey

Be certain that the ends of hard lines are squarely seated in the
wheel cylinders and are tight.

Here, nothing beats having a MityVac (hand-activated vacuum
pump) for pulling hydraulic fluid through the system. ~ $30.

It makes brake bleeding mostly a one-man operation.
Daniel Wong

This thread was discussed between 29/09/2011 and 01/10/2011

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