bleeding brakes

Technical MGB discussion
Not_Anumber
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Forename: Chris
Surname: Silk

bleeding brakes

Post by Not_Anumber » Mon Sep 28, 2020 4:23 pm

Im reassembling my 1979 MGB that a friend had stripped down before a series of health issues stopped him in his tracks. I therefore have the fun task of trying to put something together from a stack of parts that i didnt take apart so am reliant on the Haynes book and the experience of others.

Ive refitted the brake master cylinder which has been in a crate for the last couple of years. I started bleeding the system and fairly rapidly got rid of the old residual fluid from each wheel - but then got stuck trying to expel the air and getting the new fluid moving through. After an hour of that I got cold and gave up for the evening. I used a combination of opening the bleed valve whilst someone pushed down on the pedal then closing before the pedal was released and using a one man device (this kept falling off the small rear bleed nipples so I stuck with the manual method for these).

What is the advice here. What is the easiest way of getting this air out please as it's not shifting ?

Ian Fozzard
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Re: bleeding brakes

Post by Ian Fozzard » Tue Sep 29, 2020 8:40 am

I think we need a little more information. Are you not getting anything through to the rear bleed nipple, or are you getting a mixture of air and brake fluid?
If nothing at all(which is what your post suggests), then I would initially suspect your master cylinder. If it has been left for a number of years the seals may well have suffered. Adding fresh brake fluid can sometimes free everything up including allowing fluid past the seals!
There are pressure brake bleeders available which will force fluid through the system, but of course may mask a leaky master cylinder. I've never had to use anything other than the simple method you describe, but did replace the master cylinder after a period of not using the BGT. The brakes would not bleed, but a new master cylinder cured that problem right away.

Ian F
1972 BGT, Blaze, Navy trim, recessed grill
1961 Midget, 948cc, Clipper Blue, Blue trim and weather gear

Tom Ward
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Re: bleeding brakes

Post by Tom Ward » Tue Sep 29, 2020 12:57 pm

Hi, As it's a later car. I presume you have a brake servo?

Some years ago my 76 car was impossible to bleed manually and I found the rubber bladder in the servo had perished so take the cover off this and have a look as well as checking the Master Cylinder.

The servo adds complication into the system which does make it hard to bleed manually - even if it's in good condition. I use a Gunson Pressure Bleeder which takes pressure from a spare wheel/tyre. Even if you succeed doing the brakes, when it comes to the clutch you will want a pressure system!

Not_Anumber
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Re: bleeding brakes

Post by Not_Anumber » Tue Sep 29, 2020 2:58 pm

The old residual fluid was easy enough to bleed, plenty of pressure there so the master cylinder seemed to cope with that.
But after that all went through it is now proving much more difficult to expel the large amount of air in the pipes before it gets back to fluid again.

Do I for instance need to open the unions at the master cylinder itself to bleed the master cylinder itself ? At the time I just connected it up, filled the resevoir and started bleeding at the wheels. Ive since read about something called bench bleeding which seems to consist of bleeding it at the master cylinder itself from what i can work out, though no idea if that would make a difference to being able to shift all this air in the pipes.

Ian Fozzard
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Re: bleeding brakes

Post by Ian Fozzard » Tue Sep 29, 2020 3:15 pm

It seems then that you pumped the master cylinder dry when first doing the job, then probably allowed air into the system? Always better to keep the master cylinder topped up throughout.

I've fitted new, dry, master cylinders, and managed to drive the small amount of air through by conventional bleeding methods.

The problem as you will know, is that air is compressible, and it is possible that all you are doing is compressing trapped air. You could try removing the bleed screw and therefore allowing freer movement, and press the brake pedal rapidly for several strokes to see if that blows it through. Then you could bleed the small remaining amount of air in a conventional manner.

Failing that, a pressure bleeder may be your best option.

I've never had to bleed the master cylinder on the bench or separately at all.

Ian F
1972 BGT, Blaze, Navy trim, recessed grill
1961 Midget, 948cc, Clipper Blue, Blue trim and weather gear

Not_Anumber
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Re: bleeding brakes

Post by Not_Anumber » Tue Sep 29, 2020 5:44 pm

I was careful to keep the reservoir topped up so it wasnt able to run dry. The fluid level in the reservoir dropped as expected whilst I was at the stage of expelling the old residual fluid at each wheel but then the level hardly moved from the point i got to the air.

Ive bled systems on several classic cars after replacing a section of pipe, a flexible hose, a caliper or rear wheel cylinder and never run into this sort of airlock situation before. Last year I changed the back axle on my capri with a new flex hose, crosspipe, 2 wheel cylinders but I had that fully bled and back on its wheels in around 20 mins. Im racking my brains trying to think why this one is so much more difficult as the B is such a similar brake system layout. All i can think of is maybe it is the presence of a large amount of air in all the pipes at once which is proving harder to pump out than just 1 pipe being full of air at a time.

Are you suggesting that I keep the bleed nipples open whilst I pump the pedal rather than shutting off the bleed nipple after every downstroke as would be done with normal brake bleeding. Would this push it through much faster or could it just suck an equal amount of air back into the pipes ?

Ian Fozzard
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Re: bleeding brakes

Post by Ian Fozzard » Tue Sep 29, 2020 6:14 pm

This is unusual, I certainly didn't mean to imply that you lacked experience or ability!!

What I was suggesting is that by removing the bleed nipple you would allow free movement of the air in the pipe. If you have the bleed nipple slightly open and a brake bleeding valve of some type fitted, then it would inevitably provide a bit of resistance. If you have an assistant with a nitrile glove, then leaving the pipe open on the pressure stroke, and putting a finger over the end for the return stroke, should give the greatest chance of driving the air out. I generally have my other half doing the pedal work whilst I open and close the relevant bleed screw - communication is essential!!

Ian F
1972 BGT, Blaze, Navy trim, recessed grill
1961 Midget, 948cc, Clipper Blue, Blue trim and weather gear

Vic Butler
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Re: bleeding brakes

Post by Vic Butler » Tue Sep 29, 2020 9:02 pm

A vacuum bleeder is supposed to be better than a pressure bleeder.
I successfully bled the brakes on my 1977 BGT which has dual circuit brakes too using a pressure bleeder.
By using a pressure or vacuum bleed there's no need to touch the brake pedal therefore the shuttle valve in the master cylinder doesn't have to be slackened off and retightened when the bleeding is complete.
A school of thought is that because the brake master cylinder piston never travels the full length of the bore unlike the clutch, then the untravelled part can become pitted on older units and when the piston travels the full length as when bleeding there's a possibility that the seals can be damaged.
1977 Stage 2 MGB GT
1975 SWB Series 3 Land Rover with a later 2.5 petrol engine

Ian Fozzard
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Re: bleeding brakes

Post by Ian Fozzard » Wed Sep 30, 2020 8:54 am

Ah! Shuttle valves and dual circuit brakes!! I have no experience of these as my B is a 72. Perhaps Vic has highlighted where the problem may lie?
I wouldn't know if "loosening the shuttle valve" is essential when bleeding brakes the conventional way in these cars.

Ian F
1972 BGT, Blaze, Navy trim, recessed grill
1961 Midget, 948cc, Clipper Blue, Blue trim and weather gear

Robert Dickinson
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Re: bleeding brakes

Post by Robert Dickinson » Wed Sep 30, 2020 9:06 am

Have you tried wedging the brake pedal down and leaving it overnight then release the pedal slowly in the morning. If a lot of air in there it might need doing this way more than once. This worked for me after struggling bleeding the brakes after fitting a new master cylinder. When a reasonable pedal pressure was reached I finished off by bleeding each corner the normal way . Good luck hope you are successful soon.

Rob.

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