Rear brakes binding and brake lights on

[Europa73]

hi all,

I had my 2 brake boosters rebuilt.

bled the system - front and back.

Adjusted the drums so the rear wheels are rotating freely.

the brakes work well and are firm.

The issue I have is.

when I brake hard the brake lights remain on and the rear brakes start to bind.

This issue remains until everything cools down.

Any thoughts?


Cheers,

[jbcollier]

First check that there is free play at the pedal of the main master cylinder.  There is a small compensating port that is exposed only when the piston is all the way back.  This port is what releases any pressure changes due to fluid temperature change.  If that is not it, then there is a similar issue in the rear circuit's booster with probably the booster's diaphragm sticking.

[Europa73]

Thanks for the information,

The prior owner fitted a tandem master cylinder running off separate reservoirs. The car still has the dual braking system.

The issue is - there is a slight brake engagement on the rear drums (brake light is also on), just enough for them to get hot and start binding.

The brake light stays on even when I disconnect the booster. When I release the brake the brake light goes out for bout 1 second them comes back on.

The brake light even comes on when the engine is off and I pressed the brake several timed to remove the vacuum from the booster.

I have striped and checked the boosters diaphragm and thats working.

Is there a way yo test if its the compensation port?

Any advise would be greatly appreciated.

I am out of ideas and desperate to hit the road - we finally have good weather in Toronto :-(

[jbcollier]

Pump the system so that the brake light is on with the pedal released.  Crack one of the rear wheel bleeders to relieve any pressure.

Brake lights now off?  Somewhere in the hydraulic system it is not releasing the pressure as it should.

Pump it up again so that the brake light stays on.  Crack the line to the rear at the master cylinder.  If this solves the problem, then double check for free play at the pedal.  Enough free play?  Pull the master and see what's up.

If cracking the line to the rear at the master does not solve the problem, then it is in the boosters.

[Europa73]

Here is a pic of the master cylinder

[Europa73]

Many thanks!

I will give it a go!

[Europa73]

Hi jbcollier,

Thanks for the recommendation.

I did follow your instructions, please see the below.

Pump the system so that the brake light is on with the pedal released.  Crack one of the rear wheel bleeders to relieve any pressure.

Brake lights now off? [RD - yes the lights went off - but - while the car was idling the lights came back on after a few mins]       Somewhere in the hydraulic system it is not releasing the pressure as it should.

Pump it up again so that the brake light stays on.  Crack the line to the rear at the master cylinder.  If this solves the problem [RD - yes the lights went off again- but - while the car was idling the lights came back on after a few mins]   , then double check for free play at the pedal [RD - I have about 5 mm of play. is this enough?] .  Enough free play?  Pull the master and see what's up.

Hoping you may have any ideas before I pull the master C.

Many thanks,

[jbcollier]

Do you still have the boosters?

[Europa73]

yep they were both re-built and I have striped and checked the boosters diaphragm and thats working.

[jbcollier]

Temporarily bypass the boosters by connecting the in and out brake lines together.   Bleed the system and try it again.  If the problem goes away then the problem is in the booster fir the rear brakes.

[EuropaTC]

Brake lights now off? [RD - yes the lights went off - but - while the car was idling the lights came back on after a few mins]       Somewhere in the hydraulic system it is not releasing the pressure as it should.

Pump it up again so that the brake light stays on.  Crack the line to the rear at the master cylinder.  If this solves the problem [RD - yes the lights went off again- but - while the car was idling the lights came back on after a few mins]   , then double check for free play at the pedal [RD - I have about 5 mm of play. is this enough?] .  Enough free play?  Pull the master and see what's up.

I think John has it right and the problem lies with the servo/booster for the rear brake system.

If you can relieve pressure and the hydraulic switch goes off but later comes on with the engine idling but without pressing the pedal, then I would expect it's the engine vacuum causing the rear servo to move.  I wonder if simply clamping the air hose feed to the rear booster would be enough to take it out of the circuit and prevent the lights coming on at idle ?

If you're not pressing the brake pedal then I'm struggling to see how the m/cylinder could apply enough pressure to the system to lock the rear brakes on it's own. 

[Grumblebuns]

Looking at you picture, you seem to have the Nissan F10 master cylinder installed on a car with boosters. That probably makes braking a bit touchy with the boosters. Before pulling the master cylinder, try hydraulically bypassing the booster as JBC suggests. The 3/4" bore Nissan master cylinder by itself should give you excellent braking force and feel. I had the Nissan master cylinder on my TCS and the brakes felt great.     

[Europa73]

Just when you think you have the brake system finally sorted out.

I have been plagued for months with an intermittent issue with the brake light coming on and the rear drums binding.

Thanks to all your advice, I finally chased the issue to the master cylinder.

On removing and inspecting there was a tiny foreign object that was stick in the fluid return hole.

I re-assembled the master cylinder, bench bled it and EUREKA – the issue was solved – no more constant brake lights on and the rear brake cylinders retracting nicely.

BUT –

Now I have excess travel in the brake pedal, it feels like it travels half way before engaging

I have a duel brake system – and – bled the brakes per the workshop manual sequence. No improvement. I re-bled the system using the sequence furthest from the master cylinder approach and still no improvement in the pedal travel.

What was interesting – in bleeding the brakes after re-fitting the master cylinder – I had air in the rear system but no air in the front system.

I thought as this is a duel system car I would have at least a bubble or 2 in both the front and rear systems.

I confirmed that the rear drums are adjusted correctly and now out of ideas.

Had anyone out there been through the same and have a resolution to this?

Many thanks,

[BDA]

Did you bench bleed the M/C? That might be something to try...

[Europa73]

yep - did that twice and no improvement.

Had no travel in the pedal before I removed it.

Cheers,