Bleeding newly fitted disc brakes

[moggiegeek]

Have fitted ESM discs to the front, Wolseley 1500 to rear, these rebuilt with new cylinders. Re-rubbered existing brake master cylinder as it was in good condition. The top hat was omitted from the rebuild. The car has a servo, but installed at correct angle.
Now finding it really difficult to bleed brakes to get firm pedal. Pedal does pump up but not convincingly.
Advice welcomed. I've used conventional bleeding and borrowed a pressurised Sealey set up. Results remain the same.

[Sleeper]

Opinions differ on this , slow and steady or quick , I prefer quick as any air trapped in the lines will not have time to flow back to the high-point.
Also raising the side/ end being bled helps remove the trapped air.

P.S. used a modified master cylinder cap and cheap garden sprayer to bleed mine....worked a tread.

John ;-)

[paul 300358]

It may be a daft suggestion but you have fitted the callipers the correct way round with the bleed nipple at the top?

[moggiegeek]

Yes, calipers right way up with bleed valve at top.
Could loosening brake pipe connection to servo whilst pressurising the system help - just cracking and then retightening the seal like bleeding a radiator?

[Sleeper]

Missed that , servo fitted at right angle....but is the air valve at the right position?? ( pointing downwards ).

John ;-)

[Steve Phillips]

Hi

Try clamping off the rear flexible brake line with a line clamp or carefully with some mole grips, this is provided you do not have stainless braided lines (these must never be clamped)

With the rear line clamped off so no fluid can pass in either direction try the pedal and see if it is now OK, if not try bleeding it out the fronts as normal, do you now have a pedal?

If no clamp off the two front flexible lines, do you now have a pedal? If no crack the unions (one at a time) on the front inner wings where the flex lines join the hard lines and bleed out here, do you have a pedal? If no it's a bad master cylinder,

If clamping off the rear line got your pedal back, then its likely to be the springs in the drums pulling the cylinders in when you lift your foot off the pedal which you then have to pump out with repeated pumps to get any sort of pedal, and as soon as you stop the springs pull the cylinders in again and your back to square one,

It's easy to resolve if that's what it is, please keep us all posted as to how you get on.

Steve

[oliver90owner]

If clamping off the rear line got your pedal back, then its likely to be the springs in the drums pulling the cylinders in when you lift your foot off the pedal which you then have to pump out with repeated pumps to get any sort of pedal, and as soon as you stop the springs pull the cylinders in again and your back to square one,


Applying the handbrake, to fix the shoes in position tight against the drums, seems to be easier than clambering under the vehicle?

[kennatt]

as soon as you stop the springs pull the cylinders in again and your back to square one,

Thats why the top hat washer is fitted to standard brakes ,to maintain a minimal positive pressure to keep all of the shoes in slight contact,thereby enabling firm short pedal traveI . I know several owners who have converted to discs with servo minus top hat,and then have a longish pedal travel .Sometimes things are best left as the original.But each to their own.

[philthehill]

The low residual line pressure 8lb/sq.in is there to ensure that the rubber cups of the wheel cylinders are kept expanded against the cylinder walls.
The top hat seal also prevents fluid pumped out from the (master) cylinder when bleeding from returning to the (master) cylinder, thus ensuring a fresh charge being delivered at each stroke of the pedal.
The residual pressure is no good for the disc brake operation as it would keep the pads constantly against the discs leading to overheating and rapid wear.
To return to having residual line pressure (with discs fitted on the front) a low pressure (check) valve can be inserted into the rear brake feed line. This will keep the wheel cylinder seals against the cylinder walls and will greatly assist in the bleeding process.
https://www.ebay.co.uk/itm/233735626956 ... SwX~BfevJA
I would suggest that the 2lb valve would be too low to be effective unless you wanted to have additional residual line pressure serving the disc brakes or had disc brakes all round.
There has been some discussion on here regarding the check valve - use the search facility to find out more.

[moggiegeek]

Thank you all for the contributions. I've now arrived at a position where the pedal has a small amount of travel before firming up. This happens with rear brakes fully adjusted up to the drums and a variety of bleeding strategies. Finally it was the patient assistant and the up/down/ close/ return/ repeat method.
I did keep the engine running to keep servo active and bled input and output from the servo.
I think I'll now be investigating Phil's last post re pressure valve in the line to the rear brakes.
Enough bleeding for now......

[moggiegeek]

Looking at check valves on line. I can see 2lb, 10lb or higher pressure adjustable types.
As Phil says residual pressure is 8lbs so would fitting a 10lb valve to the rear drums be close enough?
I couldn't find any further threads - has anyone tried this route with a mix of discs and drums?

[kevin s]

We have discs/drums on ours with a beetle dual circuit mcyl and in line servo on the front circuit only, had to ezi bleed it initially (all new components) , and now the pedal travel is minimal, there's no residual pressure valve in the circuit. We did seem to need to adjust the rears fairly regurlarly initially but now they seem to have bedded in and haven't needed doing for a while (though the handbrake cable has stretched and needs adjusting now).

[moggiegeek]

Always more to learn!
Still feeling brakes slightly less than optimal firmness checked car over B4 test flight and discovered no brake lights.
Odd coincidence and I guess as system dismantled air might be trapped in brake switch?

[kevin s]

As all pressures in the system will be equal the brake switch should work even if there is air trapped under it. Check the wiring first (join the 2 wires at the switch and the lights should come on) but there are a lot of dodgy switches out there.

[moggiegeek]

Final result.
Re brake light switch:
Turned out to be a defective switch. Strange timing to go but replacement solved the problem.
Re pedal hardness and travel.
Replaced master cylinder with new - not a copy - despite fitting it and forgetting to remove top hat seal when I had done that and bled through under pressure the pedal is brilliant.
I've learned lots of bleeding strategies. Thanks to all.
A question
Would you fit a 10lb return valve at the beginning of the line to rear drums?

[paul 300358]

I must admit that mine is still on the shelf and I will fit it the next time I do anything that requires the brakes to be bled. You can fit it anywhere on the pipe running to the back brakes, obviously forward of the rear axle. Make sure that you fit it the right way round.

[moggiegeek]

I did read that these valves are best fitted as far away from the drum brakes as possible. Not sure of the logic of that.
Arrow should point to master cylinder?

[kevin s]

If the pedal is now fine I would not bother with a redidual pressure valve, just something else to go wrong.

[paul 300358]

Arrow should point to master cylinder?

Mine is the opposite, the arrow denotes the direction of flow, so towards the rear axle.