rubbish rear brakes

[gaz_moose]

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

i put the new cylinders on last time. although i cant be sure that it actually met the requirement last year as i left the vehicle with them, whereas this time i saw them do it and they called me over to show me the crappy results on the brake tester.

the front brakes seem ok, they provide 650kg braking force (or what ever the test is) the rears score about 40-50. im told they should be more like 150.

ckjdjk

i set them up as you described. i checked the cylinders were moving with an assistant.
i clamped all 4 rear hoses and still had a long pedal so bled front and rear brakes with the engine running. air did come out a couple and the old fluid was a bit dirty but not real bad. pumped through just under a litre of new fluid overall.


i will have to jack it up again tomorrow to see if the handbrake is still rubbish as the garage never tested this today as they guy was in an arsey mood.

 

[gaz_moose]

I hope that's a OLD picture ?

that's the old ****e that went in the bin last year the drums were fairly worn/scored also. when i fitted new shoes and cylinders.

this year i found i had 1 slipping and 1 seized adjuster and it was cheaper to buy shoes/springs/adjusters as a complete set than to just buy the adjusters, so i fitted all new again and new drums. hence my disappointment that all new stuff hasn't solved it.

im tempted to bypass the load sensing valve with some 'T' pieces and just fit a manual pressure bias valve i have kicking round off an old race car. i have 2 brake lines going to the rear so i guess its 1 per side.

 

[wookie]

Can you disconnect the lever on the load sensing valve from the axle and then tie it up at maximum load position and then try the rear brakes again ? They should be a lot better.

Also try pumping the brake pedal enough times with the engine off to use up all the vacuum assist, then press and hold the brake pedal down and then start the engine. The brake pedal should go down a bit more as the vacuum assist helps you if it doesn't that's something else to look at.

It could also be the drum is too worn and therefore oversize enough that the brake shoes can't reach it to press on it properly.

 

[gaz_moose]

the MOT guy never checked the handbrake in the machine today so i don't know the official results.

my brain is fried from what i have done and in what order etc (overthinking it!!).. so i will start afresh tomorrow and take notes.

you guys have given me some good things to look for/try so i will give them a bash and see what happens.

i will update tomorrow. cheers guys.

 

[carbon]

I assume this vehicle is class 7, if so are you taking it for test with a load on it. The garage I go to like class 7's to have some load on them to get some brake effort from the rear axles due to load sensing valves. Have taken my truck for mot today and dvsa like to have at least 65% axle load to get accurate results.

Not sure if this helps at all just found this in the MOT inspection manual for class 3, 4, 5 and 7.

"When testing service brake performance on unladen Class 7 vehicles
premature wheel lock can occur resulting in less than the required brake effort
being achieved.


This may be due to the action of load sensing or pressure reducing equipment
in the service brake system.
In such cases, the service brake percentage efficiency is considered
satisfactory if:

(i)
more than half the wheels lock, or
(ii)
both front
wheels lock and at least 100kg (220lb) is achieved by each
rear wheel, or
(iii)
for three axle vehicles; both front wheels lock and at least 50kg (110lb)
is achieved by each rear wheel
.
If after applying the above criteria
the minimum brake efficiency is still not met,
then a decelerometer test should be carried out."

 

[8ob]

If after applying the above criteria
the minimum brake efficiency is still not met,
then a decelerometer test should be carried out."

Enter the Tapley meter If it cant pass with one of those there is no hope, just letting your foot off the accelerator is normally enough. They get used on our 4x4 a 6x6 trucks because they get spat out of the rollers.

Bob

 

[gaz_moose]

yes, class 7 3500kg. three axle

i was using the formula of brake efficiency divided by vehicle weight multiplied by 100 = percentage. i think the foot brake has to be above 50% and the handbrake 16%

i was getting about 40kg for each rear wheel on the footbrake. he never tested handbrake which is annoying.

i don't think the load sensing valve does much on this truck as the heaviest car i have is an astra which weighs about 1200kg and if i stick that on the back the suspension on the truck only drops 5mm or so.

 

[carbon]

Could make a difference, but not sure id want to keep taking a vehicle for retest to test if it did, but suppose it depends how well you get on with the garage doing the testing

 

[dannyp]

crap brake shoes would be my bet. i dont know what the friction material is on some cheep pattern shoes but it dosent do friction very well. the other option for cheep shoes is the stuff with so much metall in it it chomps through the drum or disk at a fair rate but can lock up anything till they get warm

 

[gaz_moose]

I disconnected the brake linkage at the front so all the cables went slack, then adjusted all the shoes so they were just binding in the drums. refitted the brake linkage and tweaked the cables to suit and now have a decent enough handbrake. using a 3 foot bit of box section as a pry bar wedged between the wheel studs I can just about turn the rear drums round using my arms, the middle pair lock solid although I could move them by standing on the bar but I weigh like 11 stone.

with the engine running and a length of wood between the pedal and steering wheel to jam the pedal down I could turn the drums round.
so I used the trolley jack to activate the load proportioning valve fully and re tested. the centre axle locked solid and the rear axle was able to be rotated using brute force.

 

[gaz_moose]

 

[gaz_moose]

I don't think the suspension compresses enough to provide enough travel to activate the thing.

im not clued up on load valves, at the front under a rubber dust boot I seem to have an allen head bolt (I point to it in one of my photos). what does this do? im tempted to wind it out and fit a penny washer then wind it fully in to fully activate the valve.

I did think that the two holes lined up when fully activated so I planned to stick a split pin in but they are quite far off.

I cant see any form of adjustment, the lower fittings are the IN and the upper are the OUT which are then T'd off each side.

 

[dobbslc]

If you wind up that valve you might end up facing the way you came th first time you hit the brakes in the wet unloaded!
As others have mentioned check if the rear cylinders are the right size, maybe find out who did the conversion and ask them. I would think if they've put two extra cylinders in you would need 4 smaller ones???

 

[carbon]

As the brakes were better with the jack it seems like the load sensing valve is working. Can it not be adjusted to work with your Astra on the back. There usually is an adjuster fitted, quite possibly the bolt your pointing to. Then take it for the mot that way. Quite surprised the test station didn't suggest adding some load to improve the brake performance

 

[wookie]

I cant see any form of adjustment, the lower fittings are the IN and the upper are the OUT which are then T'd off each side.

Often it's either the load sensing valve is mounted on slotted holes so you move it up/down to adjust or the adjustment is in the arm coming off the axle.

 

[gaz_moose]

I will see if I can get the valve to stay full ON just for the MOT then return it to how it was. i don't want the rear wheels to be able to lock up on the footbrake ever....



I did some investigation into brake cylinder sizes and found that the master cylinder has a 25.4mm bore

the original rear brake would have had 42mm pistons each side so 84mm total area.

I measured the piston size on one of the cylinders i have fitted and it was about 18mm. 2 pistons per cylinder and 4 cylinders in total = 18mm x 2 x 4 = 144mm (which would give me MEGA brakes, but this is not the case) or would it not work like that and it should be 18mm x 4 = 72mm

i actually have another pair of ford cylinders on the shelf that would fit so i measured one of those and it was 20mm which would give me 20 x 2 x 4 = 160mm or.. 20 x 4 = 80mm

messing around on the eurocar parts website suggests i can also get 22mm cylinders.

if i remember correctly the larger the cylinder the larger the pressure/braking force but the longer the pedal. a longer pedal is not wanted.

 

[colnerov]

Hi, You're on the right track but you need to compare the area of the pistons not the diameter. Now we'll see whether you were paying attention at school, or you could use online calculators to get the area from the diameter.

Colin

 

[gaz_moose]

if it was just 1 drum that wasn't playing along I would agree on changing a flexi but for all 4 of them to go bad would be terrible luck. I did give them a good look over whilst the brake pedal was held down and they looked normal. I put a clamp on every rear hose and still had a long brake pedal which I though strange. the front brake pads are about 50% worn.

using the formula for area of a circle, Area = Pi ((d/2)^2)

for the original disc brake callipers that would have been fitted originally that had a single 42mm piston per side, I get,
Area = 2770.88mm

for the wheel cylinder currently fitted (2 18mm (ish) pistons per wheel cylinder)
Area = 2036mm for all 8 pistons.

if I fit some 20mm piston cylinders
Area = 2513.6mm which is more like it.

I can get 22mm piston cylinders but these give 3041mm

but im wondering how much piston size affects brake pedal travel.

 

[colnerov]

Hi, With a larger area piston you get more force but less travel, so although the piston pushes the shoes harder against the drum you get a longer pedal because the of the distance the piston has to travel.

Colin

 

[gaz_moose]

what's got me confused though is that when I clamped off all of the rear flexi hoses I still had a long pedal. which is strange as clamping them off would stop the rear brakes from operating.

that's why im scared to fit the larger rear cylinders and get an even longer pedal.

the front discs/pads are a bit worn but that shouldn't be an issue surely.