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MG MGB Technical - Brakes, you win!

I recently posted a question about bleeding my new brake system and got some excellent advice. Many thanks!

Sadly, I'm not much better off! I have a completely new (lines and all) single line brake system in a '67.

I cannot get away from the spongy pedal. I've tried gravity, Ezibleed and my wife pumping the pedal. I've just borrowed a Mityvac from a neighbor, that's next.

I have no leaks. I'm beginning to think something else is wrong.

Is there anything else that might give the appearance of air in the lines? Or am I just facing a particularly annoying air bubble?

Thanks in advance for any suggestions.
Dave
David Steverson

Not sure what you meant by "gravity" - it may be the same as my suggestion.

I unscrewed the cap from the master cylinder (allowing it to lay loosesly on the cylinder to keep the muck out), then wedged the brake pedal all the way to the floor (I used a 2x4 with padding at each end). I left it that way for 2 days (probably overkill). Had a nice firm pedal afterwards. My dad was the one who told me about this trick - I was skeptical too...

Linz

(Don't forget to disconnect either the battery or the brake lights - for obvious reasons)
Linz

'lo David,

Have you adjusted your rear brakes? If they're set too loosely you'll have quite a bit of pedal travel before they take up. Easy way to tell is to put the hand brake on firmly while pressing the pedal, then keep the handbrake on and see how the pedal feels. If it's firm, you've found your problem.

ttfn,
--
Olly S
Oliver Stephenson

Linz,
On the gravity I just used a clear bleeder hose and let it sit there until no air could be seen. I did not try it with the pedal to the floor. When you did that, did you have a hose on each wheel? How did you keep the MC full?

Olly,
Yes, I have thought of the rear brakes bit. I know pedal height is controlled by that sort of thing, but I'm having trouble with the pedal being very spongy. Pumping does not improve the situation.
By the way, I have some good brummie friends in your neck of the woods!

Thanks for your input,
Dave
David Steverson

Dave,
I know the frustration and believe me you are not alone in having this happen.

A couple of things…one concerns the state of the hoses. Are they soft at all? This can give the impression of air in the system as the pedal is pressed. The hoses flex and the pedal feels spongy even though there is no air in the system at all.

The second is pretty basic but it is easy to overlook something in the heat of the doing so if I am telling you things you already know please forgive me.
Have you bled in the correct sequence; namely rear pass side first, then rear drivesr side, then front pass side and finally front drivers side.
When bleeding the way I always use is:
Fill the M/C to max. Fill a transparent container with clean new fluid to about half way up. Attach a hose to the bleed nipple and ensure the hose end is under the level of the fluid in the container to avoid sucking in air.
Person A in the drivers seat presses on the pedal slowly and evenly to the floor as person B opens the bleed screw to allow the fluid to be expelled. When the pedal is at the floor person A tells B to close the bleed screw. A then tops up the M/C if needed and this allows the pedal to return to the start position and the sequence is repeated until the new fluid comes out the bleed screw. As each wheel is finished the sequence is repeated at the next wheel in turn until all are done.

Finally have you bled the M/C prior to installing? Not doing this can cause air to be trapped in the M/C.

Good Luck, Cheers, Pete.

Peter Thomas

Pete,
My rubber hoses are all new, ideally there should be no issue there.
I have bleed in the order you mention.
I did not however, bench bleed the MC. If there is air trapped there, I wonder if there is a way to correct that at this point?
Thanks, Dave
David Steverson

Rather than your wife pumping the pedal have you tried rapidly opening and closing the front nipples while she stands on the pedal? Both my cars need that after Eezi-Bleeding.
Paul Hunt

Paul,
I'll try that one tonight. Thanks, Dave
David Steverson

I completely replace the brake & clutch system on My A--the only thing original after I was done was the brake and clutch pedals. I bled the entire thing with the Mityvac, followed by actuating the system beaucoup times and readjusting the rears, then going back to each fitting, attaching a hose and submerging it it fluid, and again actuating the pedal again several times. I had good firm pedals and have had no problems since. Coat the bleeder threads in grease, to get an airtight seal, before you start..
R. L Carleen

Have you tried to isolate the problem? I went through a rather lengthy diagnostic and finally clamped the front brake lines using vice gripes and cloth. In my case the rear brakes were fine, a good firm pedal. I then unclamped one front at a time to determine which brake was the problem. It turns out a left hand caliper on the right side does not bleed well.
Dana Elliott

Dave
The problem you describe is likely to be due to the master cylinder seal. Remove the piston from the master cylinder and replace the seal. Then rebleed using easy bleed as you have done and it should be OK.
Bob

Dave - the procedure I described is used after you've bled your brakes in the "normal" fashion. It's kind of a last step. All bleeder screws are closed and the MC cap is left loose on the MC.

You use the 2x4 to replace your leg to hold the brake pedal down (for a couple of days). I'm not sure how it works, but essentially you're keeping constant pressure to the master cylinder and any air bubbles just work their way to the highest point in the system (the master cylinder reservoir). I've also heard some folks will jack up the front of the car to speed the process (from the rear brakes).

I know - it sounded bogus to me too, but I tried it as a last resort before ripping out the entire brake system.

It worked for me and it doesn't cost anything - YMMV

Linz
Linz

Many many thanks to all for their advice.
I found the problem by isolating sections of the brake system as Dana suggested.
Turns out I have bad calipers. When I clamp the caliper flex lines, I have a fantastic brake pedal.

And, you guessed it...that's the only part of the brake system I chose to rebuild, rather than replace. Do i feel like a complete eejit !!

Oh well, I guess after a complete car rebuild, you're allowed one goof-up.
Dave
David Steverson

Dave, Glad to hear the problem is finally found and fixed.
Good to hear some feedback as well.
Cheers, Pete.
Peter Thomas

David, before you rip everything off and spend loads of extra wonga, did you fit new discs? If not, then you probably will not get a solid pedal until the new pads have bedded in to the surface of the discs.
Tim Cuthill

Another cause for the record now the matter has been resolved:
I was once supplied the wrong seal set for the M/C which on inspection fitted fine, but was a few thou out so the system would not firm up as above.
Martin

Linz...looks like our Dad's were onto something. My dad taught me the same trick when we installed a new hydraulic steering system in his boat a long time ago. After the normal bleeding exercise, he would crack the fitting at the reservoir (highest point) and leave overnight. He said that all the tiny air bubbles still in the system would begin working upstream, joining with other little tiny air bubbles into larger bubbles until they all made their way up to the open fitting (sort of a rolling snowball effect). The normal bleeding exercise should get most of the air out, but this process leaves no doubt. -Rod
Rod in NC

I think you're right Rod - maybe it's a "southern thing"...

It's nice to know someone else has used this trick - sort of adds some validity to it!

Linz
Linz

Rod and Linz, Hate to burst any bubbles, but the high point in most hydraulic systems is the reservoir. With no pressure applied, the rervoir is open to the system , to allow for return flow and thermal expansion. Tiny air bubbles will automatically float up through the system and eventually end up in the reservoir. Loosening a line and leaving it overnight allows the reservoir fluid to seep out through the line break and at the same time allows contaminants such as moisture from the air to enter the system. The theory is correct, but I would not follow that particular method.
R. L Carleen

This thread was discussed between 05/01/2003 and 09/01/2003

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