Rear brake locking!

25th April 2015, 19:57

My Saxo had brand new discs and pads just before I bought it a couple of months ago, so they've had time to bed in!
When I'm braking really hard the rear right locks up.
Obviously this isn't ideal as it could put me into a spin,
It's a vtr. Do the saxo's have a rear load sensing valve? Could it be this or does anyone else have any suggestions?
Dodgy calliper etc?
Many thanks

26th April 2015, 07:22

Hi, Im new to Saxo's too BUT have been driving for 30 years. Im my sons chief mechanic!
I have an axle waiting to be fitted,complete with brakes.
Im a natural fiddler and before I fit it I have striped and cleaned the breaks.
Easy to do,just use a trolley jack and axle stands.
The caliper itself moves on two sliders in and out when the brakes are applied. These can get 'stuck' up with old grease,brake dust and crud which effectively binds the caliper in place.
When the sliders are removed I clean them up using a combination of brake cleaner and a brass wire brush drill attachment.
Re assemble with RED brake grease. This is designed NOT to react with the rubber. Anything else can cause the rubber to swell and grab the slider.
Crud can also get behind the piston seals as can moisture and eventually rust. Whilst its apart check this out to BUT remember to screw the piston in and not try and force it back with a g clamp etc as you would the fronts. The rears wind in and out as part of the handbrake mechanism.
HTH

26th April 2015, 08:13

Ok thanks, I've cleaned/changed quite a few myself.
I just wondered if there was a common reason other than dirt in the calliper which seems unlikely as they are all new.
Thanks

26th April 2015, 10:14

Quote:

Originally Posted by W4rdy1

Ok thanks, I've cleaned/changed quite a few myself.
I just wondered if there was a common reason other than dirt in the calliper which seems unlikely as they are all new.
Thanks

Thought it was only the discs and pads that were new. Oh hum.

3rd May 2015, 14:51

I've been looking into this problem again today.
The rear right is still locking first but if I brake harder still, both rears lock up.
This suggests to me that it could be a bias thing.
I've looked into the brake bias controller thing and my car doesn't seem to have them fitted.
Is this common on a certain age of Saxo or has someone messed about with the car?
Any help appreciated!

3rd May 2015, 15:06

Brake bias valve, might be faulty or missing one like me. Rear locks up under heavy braking, definitely worth a check.

3rd May 2015, 17:14

There's always going to be slightly more pressure to one wheel than the other.

The thing is, it's also to do with weight distribution, you could have a tiny bit more weight over the left rear wheel than the right rear, so the right will lock first.

3rd May 2015, 17:15

basically what I'm saying is I don't think there's a problem here at all. although the rears should NEVER lock up before the front. fit a manual bias valve.

3rd May 2015, 21:01

So is it a common thing for the rears to lock before the fronts?
Surely not as it's quite dangerous!
Should my car not have bias valves fitted?
I read a post about swapping a drum rear beAm with a disc rear beam, and something to do with pressure restrictors needing to be fitted with discs?
Can anyone shed any light on this?
Thanks

5th May 2015, 14:30

Can anyone help?

5th May 2015, 17:17

I told you already, the rears should never lock up before the front.

However, because the cars are so light, standard setups do allow this, it is not dangerous as you should never be that hard on the brakes with road use.

5th May 2015, 20:03

Well you have said two things!
'It should never lock the rears before the front', and
'It may lock the rears before the fronts as standard.'

Agreed that you shouldn't need to brake that hard during everyday driving,
However, I did need to the other day when someone pulled out in front of me and it locked up.
I can't just not brake hard in an emergency situation, and this is the reason I have come on here to sort the problem.
Thanks

5th May 2015, 21:39

Quote:

Originally Posted by W4rdy1

Well you have said two things!
'It should never lock the rears before the front', and
'It may lock the rears before the fronts as standard.'

Agreed that you shouldn't need to brake that hard during everyday driving,
However, I did need to the other day when someone pulled out in front of me and it locked up.
I can't just not brake hard in an emergency situation, and this is the reason I have come on here to sort the problem.
Thanks

Yes one was a definite, the other said "may"

Models with 2 port master cylinder should have a load sensing valve.
I would imagine that non ABS models with disc brakes on the rear should have one too.

Converting from drums to discs is different, forget anything anyone tells you about that.

Get under the back of your car and look for the load sensing valve. it should be on the passenger side near the shock absorber top mount.

The standard brakes are calculated, this means you need 4 identical tyres and the OEM size. If not, then expect the bias to be wrong.

5th May 2015, 22:14

I haven't looked if I have a two port master cylinder yet.
As in post #5 I said that I am missing a load sensing valve.
I came to the conclusion that the rear right locking was just an imbalance across the axle that's why I started looking into the load sensing valve.

I'm now thinking I need to fit a load sensing valve to mine which I think would mean new brake lines from the master cylinder to bias valve, as currently the go through a solid bracket to flexi's then to the callipers.

6th May 2015, 15:39

Quote:

Originally Posted by W4rdy1

I haven't looked if I have a two port master cylinder yet.
As in post #5 I said that I am missing a load sensing valve.
I came to the conclusion that the rear right locking was just an imbalance across the axle that's why I started looking into the load sensing valve.

I'm now thinking I need to fit a load sensing valve to mine which I think would mean new brake lines from the master cylinder to bias valve, as currently the go through a solid bracket to flexi's then to the callipers.

IF your going to retro fit a bias valve you could chop/cut the brake line near the right rear wheel area and make the connector to fit. I changed the rear axle last week and cut the original pipes off the bias valve as they were seized solid.
Remade with nice flexible copper.

7th May 2015, 08:08

Quote:

Originally Posted by cairoc

IF your going to retro fit a bias valve you could chop/cut the brake line near the right rear wheel area and make the connector to fit. I changed the rear axle last week and cut the original pipes off the bias valve as they were seized solid.
Remade with nice flexible copper.

Are the current brake lines steel?
I thought about sourcing the bias valve from a scrapped car and taking the existing piping with it.
Maybe cut it where it straightens out and put a join in to the existing brake lines.
How hard is it to swage a steel pipe flange or did you join them some other way?
Many thanks

7th May 2015, 08:32

yup they are mild steel. I havent got the tools tbh so i paid a mate £25 to do the job. He put a join on the line ,even so it took him over an hour.

7th May 2015, 09:48

Quote:

Originally Posted by cairoc

yup they are mild steel. I havent got the tools tbh so i paid a mate £25 to do the job. He put a join on the line ,even so it took him over an hour.

Ok thanks, if you get a chance to speak to him at all, would you mind asking how he joined them?
Don't worry if you can't!
Thanks

2nd June 2015, 19:38

I've been looking back into this again now the car has passed an MOT and I know it's worth keeping.

My only concerns are, the bias valves don't seem to come with any unions in them?
Does anyone know what I would require to make them fit onto some brake line?

Thanks

2nd June 2015, 21:07

standard unions that you get from any motor factor, pretty sure you need two different sizes for the bias valve

2nd June 2015, 21:46

Yeah makes sense, ta

2nd June 2015, 22:20

Yeah the two sizes are M10x1 and M12x1

3rd June 2015, 07:44

all i,m goign to say is when i set up a car for the track iwill always have rears "just starting to lock before front --that allows you to turn in and compensate for some understeer --so it will be the bias valve or torsions bars not set equal causing uneven wieght on rear
load bias valve is onthe car cos it needs more rear brakes when loaded
eg rear suspension compressed more --
car should squat under braking not dive at front
if its been lowered then its a racing certainty that the rear bias had not been adjusted to correct new ride height

fitting a manual bias vlave --then you can adjust to taste
and on warm up lap you set it everyday so rears ""just start to lock before fronts"
you can still steer with backs locked --not so the fronts

all thse things are relative of course
corner weight the car to check if suspension is giving even weights on both back wheels -and a pair of weights at front
with in 10kgs will do

normally about 70% front 30% rear-80/20 depending how you want it to turn in

9th June 2015, 09:43

Wardy - what you said about the rear right locking up before the rear left - I have the exact same problem.
I spoke to various people about it, and the conclusion I came to was brake lines! there may be a blockage etc, but it won't be the bias valve as they are split front to back, under hard braking, if both rear lines are unobstructed, the time between one and the other locking up would be unnoticeable.

If the car's lowered they're especially prone to the rear locking up. Lop it off, redo rear brake lines, fit another aftermarket brake bias valve, that way you can control the point of lock