Technical Fiat Uno - rear wheel skid

[capers626]

Hi all,

I have a problem with the rear wheels skidding on my 1993 Fiat Uno Mk2 during an emergency stop! At least, it is more or less in a straight line rather than a complete loss of the back end (remaining rear tyre tread depth is 4mm on each side).

For some time now, I have noticed the rear brakes seem to be doing more of the work when applying the brakes, as shown by a temporary grinding noise when the car has been parked for as little as forty five minutes (not just the effect of an overnight build up of surface rust on the drums/shoes, which is to be expected at this time of year).

The old brake force regulator valve, which had seized, has been replaced and the rear shoes, cylinders and drums checked and replaced where necessary - in practice, only a new wheel cylinder or two). The skidding occurred both before and after fitting the new valve.

There are no signs that the (original) master cylinder has failed but perhaps it is starting to deteriorate and this is affecting the braking system?

The MOT testing station have offered to carry out a free brake test but not sure if this would show whether there is an imbalance between front and rear.

Any thoughts or advice would be appreciated.

Thanks,
Andy

 

[jackwhoo]

What engine size?
Abs or no abs

Mot brake roller tests can show brake force on front and rear wheels , they have offered get their opinion.

If non abs Are you saying the rear wheels lock up before the front wheels ?

The link to the brake force regulator may need to be adjusted- slackened off slightly.

 

[jackwhoo]

You are not pulling on the hand brake during emergency stop ?

 

[capers626]

What engine size?
Abs or no abs

Mot brake roller tests can show brake force on front and rear wheels , they have offered get their opinion.

If non abs Are you saying the rear wheels lock up before the front wheels ?

The link to the brake force regulator may need to be adjusted- slackened off slightly.

Hi Jack,

It's a 1.1 litre engine, non ABS with brake servo. I will see what the brake test shows up. Also, get the brake force regulator link slackened off slightly, as you suggest, and then report back.

Cheers,
Andy

 

[capers626]

You are not pulling on the hand brake during emergency stop ?

No, just applying the brakes as you would in an emergency stop situation.

 

[jackwhoo]

Plus check front brakes working correctly.
If front weak rears will appear stronger.
Please keep us up to date (-:

 

[The Panda Nut]

What engine size?
Abs or no abs

Mot brake roller tests can show brake force on front and rear wheels , they have offered get their opinion.

If non abs Are you saying the rear wheels lock up before the front wheels ?

The link to the brake force regulator may need to be adjusted- slackened off slightly.

Agree adjustemt is the most likely cause but check there are no leaks inside the drums as that can cause grabbing.

 

[capers626]

Hi again,

Quick update after the brake test was carried out at the MOT testing station.

In summary, the tester didn't, in his opinion, pick up any issues and any variances are within tolerance. Overall result would be a pass.

Delving into the details, the performance of the service brake front showed a difference between left (158kgf) and right (230kgf) so, if I understand correctly, the left one was locking up earlier than the right. This is shown as an imbalance of 19% on the printout.

The service brake rear figures were quite similar 103kgf (left) and 126kgf (right) but the imbalance stated as 50%, which I am not sure I understand.

Overall efficiency of service brake is described as 77%.

No problems with bind, judder or grab.

Recently, I encountered a sideways spin going down a steep hill when driving the car from cold and applying the brake fairly hard. On closer inspection, the two rear tyres are from 2007 and have degraded over time although tread depth is 4mm on each side.

I am due to have these rear tyres replaced later today.

Bit concerned about the difference between the front calipers, the lower figure (nearside) is for an aftermarket Rollco unit fitted just over a year ago whereas the higher figure (offside) is, I believe, the original Fiat unit. The Rollco one had a 12 month warranty which expired last month.

Could there be a fault with nearside caliper or a blockage/some deterioration in the brake lines?

Next step might be to have the brake force regulator link slackened off slightly plus check brake pipes/hoses for front (both sides).

All feedback welcomed.

Cheers,

Andy

 

[jackwhoo]

Caliper / hose problems most commonly cause brake drag rather than lower brake force(unless hose so bad it is ballooning under pressure)

 

[capers626]

Hi jackwhoo,

Thanks for the info.

In your opinion, what would be the likely causes of lower brake force on the nearside caliper?

Cheers,
Andy

 

[The Panda Nut]

I wouldnt tolerate this difference at the front its way too big and it looks like the front / rear imbalances stated are reversed. We had less thsn this on the Panda and it was casuing issues. The discs should be evenly worn and if part of the swept area is rusty you may start to notice imbalance. I changed the front calipers and the front and rear hoses and it was resolved. You may also need new discs / pads. I would certainly look at the caliper slider bushings and the piston seals. Calipers are normally an easy fix for imbalance and if you can get them worth doing, Otherwise a strip and clean and thorough bleed may do the trick. 158 / 230 is more than 20% difference. You will notice this and may experience problems. Once the front calipers were done the difference was reduced to <4% and its very hard to get things better than this without changing everything on an old car. Uno split circuit is front left rear right on one and the other does the rest. This suggests the master cylinder is OK but check it for leaks. I dont like brake hoses over 10 years old and would definitely change them at 15 years on safety grounds although many still work on for more years, why woud you want to trust such old rubber. Working the pistons right out and back sometimes helps free thimgs, but if the car had been stood, rusty lines rings form internally on the pisotn bores and pistons are likely. Sometimes this can be cleaned away with a honing tool but great care needs to be taken using such things. Dust in the rear drums may cause issues and cleaning this away is a good idea to improve things -on an annual basis.

 

[capers626]

Hi The Panda Nut,

It looks like a full brake strip and clean is called for on both front and back along with an assessment of wear rates/condition of all components on each side.

I will check service history but pretty sure the flexi hoses on the front are quite new and the intermediate one has been changed recently along with the brake regulator valve.

The nearside caliper (with lower brake force) is only just over a year old but slider bushings and the piston seals can still get clogged up, as you mentioned. Although the brake fluid doesn't look cloudy it has probably deteriorated over time due to its hygroscopic properties.

The brake report has highlighted the front caliper imbalance but the issue with rear wheel skidding hasn't gone away. Adjusting the bias by slackening off the brake force regulator link slightly is the next step to see if that makes any difference. Though perhaps sorting the caliper is best done first?

Thanks,
Andy

 

[GettinBetter]

I wouldnt tolerate this difference...

Nor me, its dangerous. I like to brake hard on occassion (don't start you lot) and hate it to pull off to the side. It has on occassion (when I first bought it) and it got sorted ASAP (luckily was just a low tyre pressure).

It looks like a full brake strip and clean is called for...

Totally agree, remember Brakes, Steering & Tyres are the only really Important things that you MUST keep in tip top condition, all the rest is a bonus.

Tyre pressures are reknown for making you pull to one side when braking (will always pull to the lower pressure side) but won't erradicate the brake test readout values. Those percentages are well out of wack, so not a simple percentage calculation, (they must have some weird and wonderful way of calculating it).

IMO, there's no other way really but to take a look....

 

[GettinBetter]

I wonder if those test readings are being caused by tyre quality? If the rollers keep turning till the tyre looses grip the wheel will lock, so if the tyres are different on each side, the tyre with the poorer grip will lock early (hence a lower force reading).

I see you've replaced the shoes on the rear, but have you closely inspected the drums? In my experience most times the rust will be removed by the shoes if it hasn't been standing too long, but occassionally if it has the rust will remove the part of the shoe in the way, which manifests itself as grooves and the rust area doesn't get polished out any longer....but grows.

 

[capers626]

The car has had four new Falken tyres fitted, including the front two before the brake test, so all should now have equal grip. The grinding noise from the rear does go off once warmed up suggesting build up of rust overnight but then reappears very quickly within a couple of hours during the day. Definitely worth closely inspecting the shoes and drums so thanks for the tip.

 

[GettinBetter]

The car has had four new Falken tyres fitted, including the front two before the brake test....

Ah, ok. Cool..

.....but not sure if this would show whether there is an imbalance between front and rear...

There is supposed to be an imbalance between front and rear, to stop them locking prematurely. so...

Next step might be to have the brake force regulator link slackened off slightly

This is a good move

 

[capers626]

Hi

I've been touch with a local mechanic and explained the brake test results. His opinion on it is that the offside brake caliper is taking longer to lock up because it is not working as efficiently as when it was new due to possible wear in the piston (or other reason). Something to bear in mind when stripping everything down!

Cheers,
Andy

 

[The Panda Nut]

Hi

I've been touch with a local mechanic and explained the brake test results. His opinion on it is that the offside brake caliper is taking longer to lock up because it is not working as efficiently as when it was new due to possible wear in the piston (or other reason). Something to bear in mind when stripping everything down!

Cheers,
Andy

Replace or fully overhaul both though if you want even braking. Doing one may throw the problem to the other side.

 

[capers626]

Point taken. I did consider replacing the offside caliper at the same time but at that stage there didn't seem to be any front braking issues. These came later when a seized brake regulator valve was discovered preceded by rear wheel skidding, which is still to be resolved.

 

[bugsymike]

Do you actually notice a bias when braking hard on the road, I mean before actually locking up any wheel, as that is down to actual grip on the road surface, tyres etc. The reason I say this is I used to Mot cars back in the late 1970s and on light cars especially I noted the weight of the Mot tester would stop the drivers side from locking until later than the passenger side if no other occupant. You could try at the Mot brake tester to have a similar weight occupant on the passenger side.
This is assuming the braking system is all good otherwise.
When testing a car that one of my customers was thinking of buying, I would warn the owner and then on a quiet stretch of road at around 40mph brake hard with hands off the steering wheel as in an emergency stop, but not to allow the wheels to skid. This would show up any out of balance, as well as frightening the passengers. Obviously non ABS systems.
Re rear brake locking prematurely, assuming all else good and no issue with compensator and rear shoes fitted correctly, I would file a slight chamfer off the leading edges of the shoes which would stop them grabbing on in "self servo action" giving them time to wear normally.
This was something I learnt at a Moskvich Dealership and necessary on brand new vehicles as well as other things to make them usable/safe? on the road. Anyone who has driven one will know what I mean.