NCT failing rear brake imbalance Toyota Starlet

metricspaces

My 1990 Toyota Starlet keeps failing NCT with imbalance in rear brakes. Wondering if people have any suggestions of areas to check? I've attached image with last three NCT reports.

First failure - replaced brake cylinder on problem brake as it was slightly leaking. Also replaced brake lines (precautionary as there was a bit of rust). New brake fluid. Brakes bled. Shoes checked, all ok.

Second failure - everything rechecked, brakes bled again to ensure no air locks, problem brake tightened up to near max setting.

Third failure - not sure what to do now!

CJhaughey

Sounds like a failed proportioning valve, its a pretty simple fix new are expensive used are cheaper but may not always work 100%

metricspaces

Thanks. Does proportioning value for this car have two separate feeds coming out of it for the rear, one for brake line going to each rear wheel?

I'm wondering if there is only one line coming out of this valve for rear wheels, then how would it cause an imbalance?

BrianD3

Which side are you calling the "problem" brake - the reading for the offside seems high for an 800 kg car. If the offside is in fact correct and the nearside should be at that level also, brake performance would be very high.

metricspaces

I'm not sure what the correct figure should be for each side. The problem side was the the side that got the cylinder replaced, cylinder on other side was not leaking.

Does a proportioning value control both rear brakes independently? Or it is one line coming out of it controlling both rear brakes?

metricspaces

I compared my current failing NCT report to my last successful one. See image below. There is too much braking effort on rear offside. Could this issue be solved by adjusting that brake on its own?

Total breaking effort on rear for last successful NCT was 2.3 kN (1.17 + 1.13)
Total breaking effort on rear for failed NCT is 2.69 kN (0.93 + 1.76)



However, when I add up the braking force at all four wheels it is pretty much identical between the two NCT reports - 6.54 kN for failed NCT and 6.49kN for last successful NCT. There is marginally more braking effort at the front in last successful NCT. So maybe the problem is the proportional valve?



It's an expensive item at almost 300e, so I want to be sure it is definitely the problem.

CJhaughey

metricspaces wrote: »

I compared my current failing NCT report to my last successful one. See image below. There is too much braking effort on rear offside. Could this issue be solved by adjusting that brake on its own?

Total breaking effort on rear for last successful NCT was 2.3 kN (1.17 + 1.13)
Total breaking effort on rear for failed NCT is 2.69 kN (0.93 + 1.76)



However, when I add up the braking force at all four wheels it is pretty much identical between the two NCT reports - 6.54 kN for failed NCT and 6.49kN for last successful NCT. There is marginally more braking effort at the front in last successful NCT. So maybe the problem is the proportional valve?



It's an expensive item at almost 300e, so I want to be sure it is definitely the problem.

Why not try and get a second hand one ? it won't cost much and will in all likelihood solve the problem
I had this on an old Avensis and a secondhand valve cured it.

metricspaces

CJhaughey wrote: »

Why not try and get a second hand one ? it won't cost much and will in all likelihood solve the problem
I had this on an old Avensis and a secondhand valve cured it.

Yes, I'm going to go with this approach. Try a second hand. Hopefully it works.

Isambard

did you check that the drum is not polluted by the leak?

metricspaces

Isambard wrote: »

did you check that the drum is not polluted by the leak?

Yep, my mechanic took it all apart again after the failed NCT, rechecked everything, rebled etc. All good. The leak was only very small he said anyhow.

But if you look at the difference in braking force at each wheel from passed NCT to failed NCT, it's not a problem with just one brake. The front brakes are down 8% and there is over 50% too much force on one of the rear brakes. A problem at just one brake is unlikely to cause a problem at all other brakes?

So I think best to just try a second hand proportioning valve?

metricspaces

CJhaughey wrote: »

Why not try and get a second hand one ? it won't cost much and will in all likelihood solve the problem
I had this on an old Avensis and a secondhand valve cured it.

Just an update on this. I got a second hand proportioning valve and finally the Starlet passed the NCT!

CJhaughey

Good to hear, thanks for coming back with the result.
It's pretty common on Toyota's of that age.

metricspaces

Yep, delighted to finally get it through the NCT. Thanks for the advice!

To answer my own question about how many lines come out of the proportioning valve - it has four lines coming out, one for each wheel.

My naive thinking was that any valve would have just two lines coming out, one for front and one for rear, that would later split off left and right. I was thinking you'd want the same breaking pressure left and right.

I'm sure there's a good reason why there are four lines coming out. Maybe to send more pressure nearside so if you slam on the brakes the car is more likely to veer away from on coming traffic?

Below are the results from the final NCT which passed.