BN1 Brakes still struggling !!

[ianh]

Having replaced all working parts (wheel and main cylinder), new linings all round, skimmed front drums and finally adjusted shoe supports they still won't work.
Having bled clear (around 7 times with many litres of fluid) after leaving over night air still appears in the pipework and bled fluid.
Could this still be air that has remained trapped in the system or is there a leak ? There is no sign of fluid leaking out, even with the brakes under pressure but I guess air could still be seeping in ?
Could it be a faulty brake light switch letting air in ?
Yours in despair

Ian

 

[nevets]

what do you mean by "they still won't work"?
spongy pedal? brakes locked up? other?
I would think that if air can seep in then fluid will seep out under pressure?

 

[Dave Russell]

Hi Ian,
A possible cause of trapped air & sometimes dragging brakes. This is pretty much a repeat of my previous post. https://www.britishcarforum.com/ubbthreads/showflat.php?Cat=0&Number=243833&Main=243187
The BN1 - BN2 master cylinders have a rather non conventional design.
The design requires that the MC piston be COMPLETELY retracted to allow back flow/venting to take place between the cylinders pressure section & the fluid supply as the brakes are released.

If the cylinder does not completely retract & vent, the brakes will remain partially applied OR any air trapped in the system will not be released back into the supply reservoir. This can obviously make complete air bleeding very difficult & also prevent complete release of the wheel cylinders.

The main cause of not venting is usually that the pedal push rod linkage is not adjusted with enough slack to allow full master cylinder retraction. Or the retracting spring is not fully doing it's job. Check this first.

A secondary cause is the rubber seal becoming slightly swelled so that the vent port is not uncovered when the piston is completely retracted. A few folks have modified the vent port in the piston to allow it to more reliably uncover when the piston is retracted. This would not normally be necessary if the seal were not swelled.

This may not be your problem, but it is worth checking.

If the previously mentioned MC bleed port is properly opening, the system should be under gravity pressure from the supply tank at all times & no air could be drawn back into the system, even if there was a tiny leak somewhere.

A sometimes cause of incomplete brake bleeding if you are using a jar & an extension hose on the bleed fitting, is air leaking back around the bleed fitting threads. Having the hose end submerged in fluid will not prevent air from being drawn back in if the bleed fitting is not closed before releasing the pedal. The only sure method is to apply the brakes, open the fitting, let fluid out, & close the fitting before releasing the brake pedal.

I wish you well.
D

 

[ianh]

BN1 Brakes Mystery is solved ? + oher insights

Hi Peeps,
For the first time in my life I finally resorted to expert mechanical assistance to sort out the Healey.

This was in form of the excellent people at Orchard Restorations in West Sussex. They have a wonderful array of Healeys in various stages of decay/restoration.

They went through the brakes with a fine tooth comb but could find nothing materially wrong with the set up apart from the routing of the inter cylinder pipework for the front wheel cylinders ( as a result of SC parts 'new' cylinders reversing the pipe outlet and bleed nipple outlet!) Pulling the master cylinder apart and checking pushrod clearance also revealed nothing.

You may remember me being indecisive whether to fit the traditional 1' front wheel cylinders or later 7/8'. I opted to keep it traditional (1') despite them obvioulsy being changed for a reason.

As a last resort these were swapped back to 7/8' and the brakes bled and worked perfectly straight off. It seems with the new master cylider there is insufficient swept volume for the larger wheel cylinders ?

Driving home I got several lean cuts from the carb. so turned back and they have a quick look at the float chamber needles which I had replaced with the ball bearing version.
It seems the SU carbs have two different heights of pedastal which hold the pivot for the float arm. Needless to say I had 1 of each and the shorter one does not allow the arm to lie close to horizontal on the needle/float resulting in incorrect operation. Orchard went into their Alladins cave and 5 mins. later we had a matching pair.

Orchard also warned that they have had huge problems with new rotor arms for the distributor. They set up a test rig and when warmed with a lighter the insulation shorts !!! Symptom is car drives for 20 mins, rotor warms and starts shorting car misfires stops and won't start for 20 mins. So be warned !!!

Final hint ensure windsreen wiper drive shafts whcih coem through the bonnet are perpendicualr to window otherwise they will wear / force off the windsreen wiper.

Hope this timely (and costly) journey of adventure benefits others !!!

 

[zblu]

Re: BN1 Brakes Mystery is solved ? + oher insights

Still doesnt offer an explanation of why you were getting air in the lines!

 

[Keoke]

Re: BN1 Brakes Mystery is solved ? + oher insights

This might be one of those strange cases Zblu where it was not a case of getting air in the lines, rather it could not all be expelled with the wrong Master Cylinder installed.--Fwiw---Keoke

 

[ianh]

Re: BN1 Brakes Mystery is solved ? + oher insights

Yep
Best theory goes the air was never fully purged out of the front cylinders although this must then have been drawn back into the master because we were still getting air out of the back too. And given the number of times they were bled you would have thought it would have run out eventually ?

Anyway I guess these old cars still have a bit of the Art as well as science in them, thats why we love them so much!
Ian