Rubbish rear Brakes (part 2)

[gaz_moose]

following on from my previous thread about the rubbish brakes on my tri-axle recovery truck,,
http://www.mig-welding.co.uk/forum/threads/rubbish-rear-brakes.74396/page-3#post-1019406

I noticed that one of my axles was seized and the other had rusted thin/cracked so I opted to buy 2 replacement axles as they were not much more money than buying new drums(the old drums were knackered). I welded MK6 fiesta rear backing plates onto them and fitted new shoes/adjusters/springs etc..

I fitted some 20mm wheel cylinders to the rear axle and some 18mm wheel cylinders to the middle axle, as I wanted to see what difference it made to braking performance.

I stuck some new discs and pads on the front whilst I was at it.

today I drove it about 10 miles to bed the shoes in a bit then took it round to the MOT centre to put on the brake tester (with a car on the back) and it measured roughly 40/50 kg of braking force per wheel on the foot brake and the handbrake. being basically a fiesta now on the rear im told it should be about 120kg.

im really struggling to see how both the foot brake and the handbrake can be so low.

 

[rtbcomp]

Servo problem?

 

[gaz_moose]

the handbrake setup is like this, the lever in the cab operates a bar that pivots off it left side which then pulls/pushes the (blue) rod back and fourth. the (red) handbrake cables all move freely (I checked and lubed them up). its 1 cable per axle with an adjuster on the one side.

 

[gaz_moose]

Servo problem?

the thing that has me stumped is that its both the footbrake and the handbrake. they score pretty much the same results on the test.

decent amounts of fluid came out when I bled each cylinder.

I think I can get 22mm dia rear cylinder which would give me more braking force on the footbrake.

I just cant see what I can change/adjust/fix etc.. that would affect both the hand and the footbrake..

I slackened the cables off so the H/B lever arms were fully back then adjusted the shoes so they were just scuffing the drums when rotated using 2 big screwdrivers to spread the auto adjusters. then tightened the cables.

the H/B 'bites' after the first few clicks.

 

[500e]

What size is the MC a smaller will give more pressure at wheels but longer travel

 

[Crater Creator]

As it's footbrake and handbrake, I'd look at the very few (albeit unlikely) things that would affect both equally.

Contaminated drums/shoes?
Shoes all adjusted correctly?

 

[mike 109444]

As it is affecting both the hand and foot brake is there ANY chance you have the shoes in wrong ?as in leading and trailing ends wrong ?

 

[gaz_moose]

google suggests that the master cylinder bore size is 25.4mm

I would be surprised if contamination is the issue as its all brand new.

its the very few things that affect both part that has me stumped.

 

[p0689109]

I agree with creator in that it is mechanical rather than hydraulic unless of course you have a hose that is acting like a restrictor due to internally collapsing!

 

[gaz_moose]

you cant really fit the shoes the wrong way round.
they are basically the same as these
http://www.pumapeople.com/forum/index.php?showtopic=101921

the handbrake cable enters the front of the backing plate then pulls the lever arm connected to the rear shoe. this pivots/acts like a cam pushing the shoe out into the drum, then it pushes (via the auto adjuster) the front shoe outwards.

im wondering if altering the pivot bar (in my H/B diagram) to increase the travel/ leverage would solve the handbrake issue then changing the wheel cylinder for the footbrake.

 

[gaz_moose]

I agree with creator in that it is mechanical rather than hydraulic unless of course you have a hose that is acting like a restrictor due to internally collapsing!

its all brand new though. new shoes/new drums, that have covered 10 maybe 20 miles at best.

 

[p0689109]

Well the bloke who took the picture reckons you can as he suggested taking a photo to ensure they go back correctly!
Shoes and drums need plenty of bedding in so perhaps 10 miles with not much brake use was not enough!

 

[mike 109444]

these pics are from my daughters 54 plate (face lift version) fiesta (not sure if that make it a mk6.
Maybe you can see something.
I did replace her shoes but found the hand brake was pants! (trying to get a lock up on rear with hand brake at 20 to 30 was neigh on impossible!)

 

[rtbcomp]

I'd expect the handbrake to be worse than the foot brake because the servo acts on the foot brake alone.

 

[wookie]

im really struggling to see how both the foot brake and the handbrake can be so low.

the handbrake setup is like this, the lever in the cab operates a bar that pivots off it left side which then pulls/pushes the (blue) rod back and fourth. the (red) handbrake cables all move freely (I checked and lubed them up). its 1 cable per axle with an adjuster on the one side.

Looking at your diagram you don't gain any mechanical advantage from the bar that pivots, it actually works against you so I'd expect a fairly short travel handbrake that is very hard to pull on and not very good, If it was only a poor handbrake I'd have been tempted to re-engineering that to improve it but since your foot brake is also poor I agree with others that you need to look at the drums & shoes.

Just out of interest what was the measured result from the front axle, that is disc so should be good.

 

[gaz_moose]

p0689109..... you cant put the shoes in wrong, the top has 'cut outs' for the auto adjusters. and the rear has a swingy arm for the handbrake.

mike,, that's basically what I fitted, 203mm backing plates/shoes to match the 203mm trailer drums. I just junked the knott-avonride stuff and welded fiesta/focus backing plates onto the axle.then fitted new fiesta/focus brake hardware. interesting to find your handbrake was also rubbish.

rtbcomp... I would have also expected the footbrake to be much better than the handbrake, hence why I was so disappointed I expected it to be 120 or higher on the brake tester.

wookie.. I will take some measurement of it tomorrow and scratch my head about some calculations on how to improve it. looking at my diagram again ive drew the pivot for the handbrake lever wrong. it should be the other side of the cable.


maybe its a combination of separate problems. either way it knocked the wind out my sails today that's for sure. pesky things.

 

[wookie]

wookie.. I will take some measurement of it tomorrow and scratch my head about some calculations on how to improve it. looking at my diagram again ive drew the pivot for the handbrake lever wrong. it should be the other side of the cable.

Glad your diagram is wrong, it just looked wrong...

 

[gaz_moose]

the footbrake is setup like in my diagram. I have 2 brake lines coming down from the ABS pump which I assume are left and right.
im confident that the back/bed whatever we want to call it is actually off a VW T4 which (from the factory) has the same size brake master cylinder as what I have fitted. I just cant figure out how to spec the rear wheel cylinders. the T4 would originally have a pair of 20.6mm dia cylinders fitted so surely when the built these and piggy backed another pair of cylinders off those it would affect how much pressure each cylinder got, then you would get half the braking force.

I was thinking along the lines of two bicycle pumps joined via a length of hose. press one ((the master cylinder)) fully down the other extends and vice versa. adding an additional pump to one of the sides would result in them both only half extending.
To get them to extend fully you would have to reduce the bore size to decrease the volume.
but....... if I fit smaller cylinders I will get less braking pressure? ie the pads being forced into the drums.

this is a new one on me ive not spec'd stuff like this before.

 

[rtbcomp]

The bicycle pump analogy doesn't hold. There is very little movement (hopefully) in a braking system and when everything comes to rest the pressure throughout the system is equal (pressure balance valves notwithstanding), so if the slave cylinders have the same cross-sectional area the force on the shoes is equal. In your diagram the pressure in all four rear slaves will be the same.

It may be well to check that the pedal hasn't reached the end of its stroke, if adding more slaves than it was designed for the extra (small) movement at the shoes might be causing too much movement in the master cylinder. Disc brakes are not prone to this problem because pads move much less than shoes.

 

[mike 109444]

Could it be an issue with the proportioning valve at the front? May be giving (proportioning) incorrect "share" of braking effort to the front and poor to the rear?
You mentioned "I have 2 brake lines coming down from the ABS pump which I assume are left and right" So does it have ABS ?? or did you mean the loading valve.