rubbish rear brakes

[gaz_moose]

i have a 3 axle recovery truck (1 axle up front 2 at the rear) the rear brakes are basically the same as a mid 90's ford escort/fiesta with single leading shoe 8" drums and it has rubber suspension axles.

I took it for MOT and it failed on the rear brakes being rubbish (both foot brake and handbrake)

I stripped it down and have replaced all 4 drums/shoes/auto adjusters/bled the brakes. and set them up by slackening off the handbrake cables and tweeking the adjusters till the pads were just scuffing the drums, then did some high speed reversing/footbraking to click the adjusters right out. but if i jack the back wheels off the floor and apply the brakes i can still turn the wheels using a crowbar.

i put it back on the MOT brake machine and they are still as rubbish as before.

So now i am at a total loss on what to try next? we seem to have some clever guys on here so i figured it would be a decent place to ask for some guidance as i am fresh out of ideas.

 

[500e]

Hoses ?

 

[steveo3002]

does it have any load valves to increase the brakes when fully loaded

 

[Ton-up]

Has it got the right master cylinder for four rear wheel cylinders? If the ms is for two cylinders (one axle) then you have effectively halved the force. (the same if your wheel cylinders are too big)
Also, has it got a proportioning valve, load sensing valve, etc

 

[8ob]

Drum brakes are self servoing, are you sure you have the leading shoe fitted in the correct place?

Bob

 

[gaz_moose]

it does have a load sensing valve. the lever arm is free to move up and down and the piston it operates looks to be going in and out of the unit so i assume it works ok although the rear suspension is pretty stiff.

in my driveway testing shenanigans i had it fully moved either way and it made little difference to the footbrake.

i did replace all of the rear cylinders last year, but didn't put much thought into the bore size of them.

 

[gaz_moose]

you cant really put the shoes in the wrong way round as the rear shoe has the (silver) lever arm for the handbrake

 

[carbon]

Is it some thing you have recently acquired or have you had it for a while and had previous mot's without issue

 

[gaz_moose]

i just remembered something. i clamped off every rear brake hose and operated the foot brake and it still had a long pedal. that's why i bled the front brakes thinking i may have had air in the system.

 

[gaz_moose]

Is it some thing you have recently acquired or have you had it for a while and had previous mot's without issue

ive had it about 3 years at a guess, it had a recent ticket when i got it and i put one on it last year.
for last years i put shoes and cylinders on it as the shoes were low and the cylinders leaking. i noted the drums were shagged but i was pretty skint so had to leave them on. they wasn't super impressive but the tester passed it so i guess they were ok.

 

[8ob]

The hand brake efficiency should tell most of the story, if thats making the correct braking force in the rollers it has to be hydraulic effort thats wrong. If its not there must be a mismatch between drum size and linings.


Bob

 

[Barking Mat]

Master Cylinder seals? Obstruction in load sensing jobee? With the drums off one by one are they all moving correctly?
Leaks?

 

[gaz_moose]

the rears bled up ok so i assume that the load sensing valve is not blocked. the foot pedal does give a breaking force, but not one good enough to lock the wheel.

when i tested them this morning prior to taking it for a test drive to 'bed the shoes in' the handbrake gave good resistance, i could overpower it with a 1 meter pry bar in wheel studs, but i figured bedding them in would make them better. ive not tested them again since (my toys got threw out the pram, so i left it)

 

[gaz_moose]

Seems i can get different bore brake cylinders, if my memory is correct a larger cylinder will give more pressure but will give me a longer pedal,, is that right?

im tempted to bypass the load sensing valve and see if it makes them better. as having a longer pedal is less than ideal.

 

[Barking Mat]

Does the pedal firmly stop before the endofits travel, or when applying full braking are you at the extent of the throw of the master cyl.

 

[gaz_moose]

the pedal doesn't go all the way to the floor or feel like it hits a mechanical stop etc.. it is still within a nice amount of pedal travel.

i had a 'normal' one of these vans before this one with a single rear axle and disc brakes and the brake pedal 'biting' point was higher in comparison. so i guess im comparing it to that one.

 

[colnerov]

Hi, The load sensing valve is there to stop the rear brakes from locking up because it can make the vehicle unstable, in extremis it can swap ends.

Colin

 

[Barking Mat]

Have you put the wrong rear brake cylinders on if last mot was OK and this one not?

 

[CKJDJK]

Check that there is not a bleed screw on the load sensing valve. Some have. You still get flow from the wheel cylinders but excess travel from the brake pedal. This would affect the brake efficiency. Check the back plates where the shoes rub. If they have ridges they can cause the shoes to not hit the drum square or even prevent the shoe moving. Check both pistons move in each cylinder. Visually check by getting helper to gently press pedal while you watch the brake operate. Do this one brake at a time with the drum in place on the other brakes and hold the shoes back with a screw driver to control the movement. Do not allow the pistons to come out to far. Check the brake cable is fully released and the brake lever on the shoe fully retracted. Then adjust the auto adjuster manually. Does the adjuster ratchet hold or slip releasing the adjustment? Check the operation of the load sensing valve. It should allow higher brake effort when moved in the same way it would move when fully loaded. Is it set up correctly with the correct springs? Is the valve actually operating or has it seized internally but the lever working externally? Check the brake hoses are not ballooning under pressure by getting helper to operate brake as hard as they can and you check every hose (all drums on). May need engine running for that to get servo assistance. Are you getting good front brakes, if they are not so good then check the servo is operating correctly. Finally check the master cylinder. If you are getting excess travel try clamping all hoses there should only be a slight movement on the pedal if the master cylinder is good and no air up to the clamps. release each one in turn and check travel again. should only be a small increase each time a clamp is undone. if you should get a big increase then the air or excess brake travel is after that clamp. Adjust/bleed that brake/s.

 

[magicgrotto]

I hope that's a OLD picture ?