Can't get the brakes pass MOT

Stock_90_TD5_SW · May 21, 2019

Dear forum members!

I was unable to pass the MOT because of insufficient brake strength. The compound efficiency was 35%, it needs to be over 50% to pass. The difference between the front and rear axle brake strength is about 3 times. The front brakes are just over the limit of acceptable, but the rear brakes are very weak.

Replaced the brake disks and pads (perforated britpart with mintex on the front axle; cheaper ones on the rear axle). Have emptied and replaced the brake fluid. The fluid level is good and is not getting lower. The callipers are said to be good, they are pressing well. The fact that the rear brakes are uniformly weak, makes me understand that the callipers must be ok and that it's something with the hydraulics.

The indie mechanic checked the system and thought that the culprit was the brake valve that divides pressure between the axles: https://www.paddockspares.com/ntc8836-brake-valve.html. Froghopper helped identify it, I ordered it and today was the second try at passing the MOT. I'm happy to say the efficiency has grown from 35% to 45%, but still not enough to pass and also not good enough for peace of mind.

I went back to the mechanic and he said he thinks it now can only be either the vacuum pump or the servo. As the vacuum pump is the cheaper one, he thought we could start there. When I press the brake pedal, I feel that it is not as responsive and efficient as could be. There is a certain feeling of dried out sponge being stepped on. Also I am unable to get the brakes lock up while driving - something that was there a couple of years ago. The only reason I suspect it could be something other than the pump or servo, is the large difference between front and rear brake efficiency.

So I would like to ask You to share Your opinions if the above stated paragraph seems reasonable to You and if I should start with the vacuum pump. As I am a bit pressed for time and delivering parts and then installing them takes at least 2 weeks, I'd like to maximise my chances at getting the brake system fixed, but don't want to replace everything immediately. Again very thankful to all of You!!!

tottot · May 21, 2019

To test if vacuum pump is working remove pipe from servo and put your thumb over the end with engine on tick over. Pipe should stick to thumb.
To test servo press on the brake [ hold it down] before starting engine. If it is working you will feel the peddle move down a bit under your foot as the engine starts.

Stock_90_TD5_SW · May 22, 2019

Thank You, tottot! We've done that before and everything works according to these tests. My question rather is what could be the logical conclusions from what I've described using the method of exclusion:
1) front brakes have 63% efficiency, rear 23%. The distribution of strength is relatively equal between the left and the right side (front 3% between left and right, left 19%, which is more but still tolerable if I understand that correctly). This tells me that the problem is with the hydraulics, not mechanics. The rear is 'chronically' underpowered.
2) we've replaced the brake valve that divides pressure between front and rear and the efficiency has grown, but not enough. Could that mean that the valve was ok and we accidentally touched something while doing the replacement? Or is the rise in efficiency a multi-factor story, so we need to replace many parts at once.
3) From what I've described, is there anything else to be replaced apart from the vacuum pump and servo, if the simple tests described by You and other forum users show that the pump and servo work? We can't say how good they work, but they definitely work!
4) So the main question is what else could be causing the problem apart from the pump and servo?

Wimblowdriver · May 22, 2019

Aged / soft flexible brake hoses. A kink in a brake pipe. Aged / dirty / moisture in brake fluid.

slomofo · May 22, 2019

You’ve put Mintex pads on Front and Cheap on rear axle? Upgrade the rear pads and get them all properly bedded in, as the setup is new the calipers will press but the pads won’t ‘bite’ unless happy.

....check for a pinch as @Wimblowdriver suggests.

kermit_rr · May 22, 2019

Are you sure the pistons in the calipers are moving freely and easily? If they are quite sticky, you'll still see movement, but will reduce performance

tottot · May 22, 2019

Do the above with rear calipers, , check for free pistons, fit quality pads and bed them in. After that I would be looking at the master cylinder .

dag019 · May 22, 2019

If you have fitted new discs and pads, they will need bedding in, I terrified myself the first time I fitted new all around, I had virtually no brakes at all! Dozen or so rolling emergency stops on a long quiet road to get everything hot and they improved dramatically.

zeaphod · May 22, 2019

65% efficiency on the fronts suggests the servo is working to me, but that is still low - 80% is possible. I'd change the master cylinder as a next step. You mentioned a spongy feel, this suggests that there is still air in the system. Defender brakes are infamous for being difficult to bleed properly.

kermit_rr · May 22, 2019

dag019 said:
If you have fitted new discs and pads, they will need bedding in, I terrified myself the first time I fitted new all around, I had virtually no brakes at all! Dozen or so rolling emergency stops on a long quiet road to get everything hot and they improved dramatically.

Definitely! A couple of hard stops. But don't actually stop and don't let the abs kick in of you have it, then drive a mile to let them cool down a bit then repeat. Don't come to a complete stop while your brakes are really hot or you'll get deposits on your discs which can give you juddering.
Of course that goes without saying in a real emergency you do what you need to do to avoid an accident!!! Bedding your brakes on isn't an excuse for running someone over!!

Shippers · May 22, 2019

kermit_rr said:
Definitely! A couple of hard stops. But don't actually stop and don't let the abs kick in of you have it, then drive a mile to let them cool down a bit then repeat. Don't come to a complete stop while your brakes are really hot or you'll get deposits on your discs which can give you juddering.
Of course that goes without saying in a real emergency you do what you need to do to avoid an accident!!! Bedding your brakes on isn't an excuse for running someone over!!

dag019 said:
If you have fitted new discs and pads, they will need bedding in, I terrified myself the first time I fitted new all around, I had virtually no brakes at all! Dozen or so rolling emergency stops on a long quiet road to get everything hot and they improved dramatically.

Never ever had to do any of the above with disk set up even when replacing all the fluid...I'd be looking at air still in system or more likely failing master cylinder but not bedding in new parts...what were the brakes like before the fail?..

dag019 · May 23, 2019

Shippers said:
Never ever had to do any of the above with disk set up even when replacing all the fluid...I'd be looking at air still in system or more likely failing master cylinder but not bedding in new parts...what were the brakes like before the fail?..

You don't need to do it for replacing any part of the brake sysem apart form new discs. Changing pads, rebuilding calipers, any part of the hydraulic system and it will perform as before. Fit new discs and they will need bedding in before you get full braking efficiency.

frog hopper · May 24, 2019

Wimblowdriver said:
Aged / soft flexible brake hoses. A kink in a brake pipe. Aged / dirty / moisture in brake fluid.

Agree with checking the rear flexi next. I think you have already changed the fluid ? but may need more bleeding.

Oldseadog · May 24, 2019

I'ds be inclined to start at the cheap end, and look at the colour and condition of the fluid.
If it's any darker shade than new, or has any floaters in it, change the lot ... and I don't mean just bleeding it, I mean give it a good flushing through.
I recently sorted out a similar probem with a friends Fender, and he now has brakes which will 'stop on a sixpence'.

Shippers · May 24, 2019

Oldseadog said:
I'ds be inclined to start at the cheap end, and look at the colour and condition of the fluid.
If it's any darker shade than new, or has any floaters in it, change the lot ... and I don't mean just bleeding it, I mean give it a good flushing through.
I recently sorted out a similar probem with a friends Fender, and he now has brakes which will 'stop on a sixpence'.

In his first post he says he has changed fluid...

Oldseadog · May 24, 2019

Heard that before, and all they did was a simple bleed when a full flush through is what's needed....

Henry_b · May 24, 2019

Check flexi's for balooning when braking, have an assistant stomp on the pedal and check all the lines.

Colthebrummie · May 24, 2019

Sounds to me like a partial blockage in one of the pipes.

Col

Shippers · May 24, 2019

Stock_90_TD5_SW said:
Replaced the brake disks and pads (perforated britpart with mintex on the front axle; cheaper ones on the rear axle). Have emptied and replaced the brake fluid.

@Oldseadog

Oldseadog · May 24, 2019

Shippers said:
@Oldseadog

"Have emptied and replaced the brake fluid"
'Replacing' does not constitute 'flushing out!', as it can leave contaminants and grunge in

Can't get the brakes pass MOT

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Think outside, no box required.

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