Car brakes conundrum

[Alex.]

Hope this is the correct place to put this, maybe someone can move it if not!

I've been helping a mate to (fail to) get the brakes working on his van, and it's reminded me of a question that's always bugged me. When bleeding dual-circuit brakes with a tandem M/C, how is it possible to pump the pedal with one bleed nipple open? In my mind that equates to a damaged circuit situation and the other circuit should still pressurise and stop the pedal going down. Shouldn't it?

Alex.

 

[gordon stephenson]

Had loads of trouble bleeding my Land Cruiser brakes a long time ago,I was failing to bleed the load limiter that attaches to the rear axle, Once I had bled that too, brake pedal solid as a rock once again.

 

[AndyStobbs]

Hope this is the correct place to put this, maybe someone can move it if not!

I've been helping a mate to (fail to) get the brakes working on his van, and it's reminded me of a question that's always bugged me. When bleeding dual-circuit brakes with a tandem M/C, how is it possible to pump the pedal with one bleed nipple open? In my mind that equates to a damaged circuit situation and the other circuit should still pressurise and stop the pedal going down. Shouldn't it?

Alex.

What you say is dead true - it gets slightly more complex once ABS gets involved.
I think it may depend a great deal on whether a vehicle has drum brakes or discs. One massive difference between the two is a caliper will always 'self adjust' to come to rest just before a bite point. The hydraulic actutor in drums - in most cases - rests in the fully retracted position; the springs on the shoes pulling it back in.
Meaning it will soak up so much more fluid to move it first to the bite point, then to actually lock the brake solid - at which point the actuator is unable to move any further.

Its important to keep in mind that each half of a tandem system does not use half of the available pedal travel - but rather that each half gives rise to half of the resistance you feel when pressing the pedal through its entire travel.

Not a definite answer I know, but the best I can offer without further thought.

 

[normspanners]

No the pedal travels further down to pressurise the other circuit coz it works on a balance between the two so you can bleed that brake circuit IE one pipe with only that bleed open.
BUT ideally you should trace the circuits some use front/ rear some diagonal splits and if abs, it will have 4 seperate circuits from the abs pump. Then to get full mastercylinder travel you open two nipples at a time so that you have full travel. I very rarely have had to resort to this as I am on my own whilst bleeding. I usually let the fluid run through the pipes naturally before I press the pedal, then only usually only slightly press the brake pedal, to pump the air out as I have had trouble in the past with master cyl sticking in the down position or with vauxhals the M/cyl destroying its self .
I wasnt saying no to you Andy!!

 

[rtbcomp]

........ In my mind that equates to a damaged circuit situation and the other circuit should still pressurise and stop the pedal going down. Shouldn't it?

Alex.

Yes. If the circuit at the front of the MC fails the plunger will hit an end stop, allowing the rear circuit to pressurise.

If the circuit at the rear of the MC fails the rear plunger will physically push the front plunger until it pressurises the "front" circuit.

http://www.flickr.com/photos/rtbcomp/sets/72157635406861228/

 

[rtbcomp]

........

Its important to keep in mind that each half of a tandem system does not use half of the available pedal travel - but rather that each half gives rise to half of the resistance you feel when pressing the pedal through its entire travel.
.........

If one half fails you will get a light pedal until it catches up with the half that is still OK, then you will get full pedal pressure.

How much pedal travel is attributed to each half depends on several things, if you have a mix of disc & drum (as Andy outlined) and the relative diameters of the hydraulic pistons.

 

[Alex.]

Thanks everyone. I begin to see the light.....