Stumped by MGA brakes

[malcolm]

I changed the rear brake cylinders in my MGA last week. There had been a problem with the master cylinder which caused the pressure to stick on and blow the seals in the rear cylinders. I replaced the rear cylinders. Tried to bleed the brakes but couldn't get pressure. The master cylinder was suspected bad anyway so I changed that.

I've been trying all day to bleed the brakes again. I've put a litre of fluid through and there is no more air coming out. Rear brakes are adjusted tight so they don't travel. I still don't get pressure. The pedal goes down to the floor and takes another 2 full travels to become firm.

Logically it must be the rear cylinders as that's when the problem started. They are funny things that require an internal spring. I wonder if the aftermarket suppliers all include the spring. Maybe I'll order some reassuringly expensive ones from one of the major suppliers.

What else can I be missing? I've never had trouble bleeding the brakes on this thing before.

 

[The_Yellow_Ardvark]

Clamp the rears flexi off.
Press brake peddle, if firm it is with rear.
Repeat for fronts to make sure.

The Master cylinder can have issues with the seals.

 

[colnerov]

Hi, Try clamping off individual flexibles to see where the problem is. Also bleed the brakes with one flexible released at a time to get faster movement of the fluid to pick up any air.

Colin
Edit: Does it have a servo? If so bleed that first.

 

[mike os]

You sure the master is good?

 

[malcolm]

I've clamped the rear flexy. Brake pedal is rock solid! Thank goodness it isn't the master cylinder as they take all day to swap.

I've got no trust in the rear cylinders. It might be there is some air in them as the bleed is on the inlet banjo bolt rather than the cylinder. But they were half the price of the mainstream supplier prices. I'll order some mainstream ones to save messing about.

 

[Pete.]

If you're bleeding oil but got no pedal then there's air trapped in the cylinder up where it can't escape to the bleed nipple.

 

[malcolm]

Could be air. I had a look at the new cylinders and they are missing the spring. I think they are returning when the brake pedal is lifted and have to travel some distance before engaging again. The ones that came off do have the spring. It's a funny set up.

 

[Justme]

Remove cylinder, clamp piston all the way in.

Hold with the bleed point at the top & bleed.

 

[daedalusminos]

Are the rears double ended (piston at each end) with a spreader spring and 'castellated' end caps? Can't you refit the original spring?

 

[gordon stephenson]

I had a similar problem years ago on a Vauxhall, With the brake shoes adjuste, It turned out that the slave pistons left an air void that would not shift. Finally managed by slacking the adjusters to push the pistons down the bore, Blead no problem after that, Good hard pedal,

 

[malcolm]

They are weird things. There is an inner piston where the hydraulics are, then a handbrake lever, then an outer piston (below in the photo) that both the inner piston and handbrake lever press directly onto.

The inner piston should have a spring behind it to stop if from backing off from the outer piston. I think that's the problem - this new cylinder doesn't have the spring so the brakes can back off and will require pedal travel before they engage. I think it is bled - tried pushing the cylinder out with the pedal while the brake drum was off which ought to have dragged some air out. It's just using all the brake pedal travel before engaging. A really bad aftermarket part that has been driving me mad for a couple days.

 

[johnik]

i had the same problem with a mini tho the springs were external, i scrapped it after not being able to get a pedal but discovered the need for springs afterwards, worth using the internal springs from you old cylinders ?

 

[tom2207]

bung the springs in and back bleed it using an old oil can , 814 merc clutch is like that ,,, you just dont get a pedal bleeding them from the top ,,, Id be bleeding the fronts too , usual furthest away starting point ,,, s3 landrovers sometimes need hanging from the tow hitch to lift the back cylinders higher than the master to get good backs ,,,, just because the masters new does not mean its good .

 

[daedalusminos]

They are weird things. There is an inner piston where the hydraulics are, then a handbrake lever, then an outer piston (below in the photo) that both the inner piston and handbrake lever press directly onto.

That sounds/looks a bit like triumph ones of the era - so the hand brake actuating lever moves when you press the foot pedal? You then need the handbrake to be correctly adjusted or it could push the piston back...which the pring is probably meant to prevent. What happens if you put the handbrake on and then apply the brakes - the first pump will take up excessive movement but what about following pumps?

 

[James1979]

Not familiar with that car but spent a lot of time on landrovers of a similar vintage years ago.

The master cylinder was a nose up affair and led to air being stuck at the top. It was a pig to bleed.

Required all sorts of pressure bleeding and back bleeding to sort. If you were unlucky it could be all day.

 

[malcolm]

That sounds/looks a bit like triumph ones of the era - so the hand brake actuating lever moves when you press the foot pedal? You then need the handbrake to be correctly adjusted or it could push the piston back...which the pring is probably meant to prevent. What happens if you put the handbrake on and then apply the brakes - the first pump will take up excessive movement but what about following pumps?

I've not connected up the handbrake. Shouldn't matter - the inner cylinder is supposed to operate directly against the outer cylinder so the handbrake lever shouldn't matter. But the handbrake lever was also the wrong shape so the handbrake would have needed a lot of adjustment to work.

The aftermarket parts on the MGA are awful. I've had better parts in the past. Guess the trick is to find a decent supplier and use them. And put up with the cost being twice as much. Would probably save money as I spent a lot on other parts trying to fix this.

I think a lot of it is down to things being done a bit differently back then and the people supplying replacement parts assuming it can be done the modern way.

 

[daedalusminos]

the inner cylinder is supposed to operate directly against the outer cylinder

Yes but the handbrake lever is captive in there - it also has a rubber boot backside which could foul the lever? I assume the whole assy is free to slide per original?

 

[daedalusminos]

The aftermarket parts on the MGA are awful.

If you can, always good to pick up some original NOS Lockheed/Girling and fit a new seal kit.

 

[malcolm]

If you can, always good to pick up some original NOS Lockheed/Girling and fit a new seal kit.

Good point. The master cylinder that came out is Lockheed and turned out to be fine. Only problem was the pushrod had rusted to the piston. I'll clean it up and smear it with red grease ready for the next swap. I've had the car for 31 years now so looks like master cylinders are due every 15 years or so. I'll keep hold of the rear wheel cylinders that worked too.

 

[mike os]

You planning on putting up some pics of the car Malcolm?