Brakes

ProcrastinatorFraser · Apr 8, 2016

1986 90, new rear slave cylinders, new master cylinder, no visible leaks in the system, yet the brakes still operate weirdly. The system was bled out a couple of months ago, so i think any air will have been pushed out the system by now, but on first start the pedal drops to the floor. A couple of pumps and the pedal comes back up to where it should be, and no matter how hard you press or how rapidly you press the pedal, it will stay at the top, where it should be. Next day, the pedal will have dropped to the floor, couple of pumps and it'll be back to normal, and so on. It's really confusing now. Any suggestions? cheers

Wimblowdriver · Apr 8, 2016

I once had a similar problem - not on a landy but - new master cylinder had a faulty internal seal, allowing fluid to seep past the piston back to the reservoir side.

ProcrastinatorFraser · Apr 10, 2016

Wimblowdriver said:
I once had a similar problem - not on a landy but - new master cylinder had a faulty internal seal, allowing fluid to seep past the piston back to the reservoir side.

I'd love to think that's not the issue, paid good money for the master cylinder

Is there a way i can test it?

Flossie · Apr 10, 2016

Clamp off the flexy pipe down by the slave and try the pedal again.
Oops!, my bad, its brakes not clutch..doh!
Same test though, clamp off all 3 flexys and try the pedal again.

zeaphod · Apr 10, 2016

I'd check your servo operation. Hose off vacuum pump, check for suction. If OK, you servo may be leaking air. Check for fluid at the master cylinder rubber boot too.

ProcrastinatorFraser · Apr 11, 2016

Flossie said:
Clamp off the flexy pipe down by the slave and try the pedal again.
Oops!, my bad, its brakes not clutch..doh!
Same test though, clamp off all 3 flexys and try the pedal again.

Clamp off? as in cut off the flow of fluid? if so, how would i do that?

ProcrastinatorFraser · Apr 11, 2016

zeaphod said:
I'd check your servo operation. Hose off vacuum pump, check for suction. If OK, you servo may be leaking air. Check for fluid at the master cylinder rubber boot too.

We initially replaced the master cylinder because fluid was seeping out of it passed the servo. The servo itself is quite recent, i checked for suction and it's fine, so how would i go about checking the servo?

Flossie · Apr 11, 2016

ProcrastinatorFraser said:
Clamp off? as in cut off the flow of fluid? if so, how would i do that?

There's a proper tool for the job, me? Never owned or used one.
Mole grips with something between the jaws so you don't damage the rubber hose.
Just enough pressure on the jaws to compress the hose, don't go mad its not necessary to flatten it!

tottot · Apr 11, 2016

To test servo press on brake pedal and then start motor,If servo working pedal will go down some more.
Does seem like a master seal problem.

jamesmartin · Apr 11, 2016

ProcrastinatorFraser said:
1986 90, new rear slave cylinders, new master cylinder, no visible leaks in the system, yet the brakes still operate weirdly. The system was bled out a couple of months ago, so i think any air will have been pushed out the system by now, but on first start the pedal drops to the floor. A couple of pumps and the pedal comes back up to where it should be, and no matter how hard you press or how rapidly you press the pedal, it will stay at the top, where it should be. Next day, the pedal will have dropped to the floor, couple of pumps and it'll be back to normal, and so on. It's really confusing now. Any suggestions? cheers

try lengthening the servo rod, hold rod with pliers whilst you do though so you dont dislodge the rod

zeaphod · Apr 11, 2016

ProcrastinatorFraser said:
We initially replaced the master cylinder because fluid was seeping out of it passed the servo. The servo itself is quite recent, i checked for suction and it's fine, so how would i go about checking the servo?

If you have suction, and the servo is recent, it "should" be holding the vacuum. I'd still be checking for a fluid leak from the master. I've re-read your original post - when you say "the next day the pedal will have dropped to the floor" do you mean it's sitting down before you try the brakes? - if so there is something up with the return springs. If not - see James's post above - he is the master!

ProcrastinatorFraser · Apr 11, 2016

zeaphod said:
If you have suction, and the servo is recent, it "should" be holding the vacuum. I'd still be checking for a fluid leak from the master. I've re-read your original post - when you say "the next day the pedal will have dropped to the floor" do you mean it's sitting down before you try the brakes? - if so there is something up with the return springs. If not - see James's post above - he is the master!

What i meant was the point where the brakes actuate was all the way to the floor, the pedal comes up just fine, it's just the point where the brakes are actually doing something

ProcrastinatorFraser · Apr 11, 2016

jamesmartin said:
try lengthening the servo rod, hold rod with pliers whilst you do though so you dont dislodge the rod

I've tried adjusting it several times, if i lengthen it any more the brakes will feel fine, but after a while the rear brakes will start building up pressure and lock the brakes on

jamesmartin · Apr 11, 2016

in which case its ok then, it would seem like master cylinder then ,presumably all wheel bearings have no movement slave cylinders arent wet in side rubbers

ProcrastinatorFraser · Apr 11, 2016

jamesmartin said:
in which case its ok then, it would seem like master cylinder then ,presumably all wheel bearings have no movement slave cylinders arent wet in side rubbers

Wheel bearings have about 300km on them, slave cylinders are fine, so i guess it's the master cylinder then? Is it possible i ordered the incorrect one?

jamesmartin · Apr 11, 2016

ive seen new calipers were piston retract given some time ie over night so it takes a pedal or 2 to push them back out, if its fine for the rest of the day its either a little air or movement of pistons id think

zeaphod · Apr 12, 2016

I'd pressure or vacuum bleed the entire system again. Even a little air can produce all sorts of odd results.

Flossie · Apr 12, 2016

That's why I suggested clamping the hoses, it helps identify which, if any, brake is faulty.

DanTheDefenderMan · Apr 13, 2016

could be master cylinder

Brakes

ProcrastinatorFraser

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Wimblowdriver

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ProcrastinatorFraser

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Flossie

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zeaphod

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ProcrastinatorFraser

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Flossie

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tottot

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jamesmartin

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ProcrastinatorFraser

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