Soft brake pedal advice

[L696ULO]

I took my car for its MOT (which it passed) and asked the garage to also change the brake fluid as the brake pedal felt soft. When bleeding the brakes they noticed that the fluid flow from one of the rear calipers wasn't flowing as freely as the others.
With new fluid in the system the pedal is exactly the same as before, the mechanic suggested this could be because of the 'slower' bleed caliper rather than the master cylinder or servo as the other 3 calipers bleed at the same flow rate.
Can anyone confirm this would be the case and where I can get either refurbished or new replacement calipers.

thanks in advance

[goldnsx]

Calipers are expensive...not a good starter.

How do the brake hoses look like?

Sometimes a bleeder screw can build up deposits which restricts the flow. Which one was it, front or rear?

[L696ULO]

Hi goldnsx

I assume the hoses are good as it passed the MOT but I'll have a look myself later. I can see how a deposit build up would cause the slow flow rate but once the clean fluid is in the system how could it affect the effectivness of the brake pedal?

The slow bleed was from the rear, I think the mechanic's idea was that although the new fluid came through it was being restricted somewhere and the main culprit would be the calliper.

[Pride]

Hi goldnsx

I assume the hoses are good as it passed the MOT but I'll have a look myself later. I can see how a deposit build up would cause the slow flow rate but once the clean fluid is in the system how could it affect the effectivness of the brake pedal?

The slow bleed was from the rear, I think the mechanic's idea was that although the new fluid came through it was being restricted somewhere and the main culprit would be the calliper.

Although the brake hoses on inspection passed the MOT it has been known that the internals of the hose can perish creating a restriction causing slow flow rates and even a one way valve effect in the most serious of cases.

[RedCarsGoFaster]

You could start with new Goodridge hoses and see if that makes a difference.

http://www.nsxcb.co.uk/entry.php?340-Goodridge-Custom-Made-Brake-Line


If you do need new calipers, R-Motion has a used pair of NA1 calipers available on eBay.

If you want to go for a full brake system overhaul, in addition to the hoses, Bigg Red will strip, recoat, and rebuild your existing calipers, though Kaz will advise you to supply your own Honda rebuild kits. Bigg Red can't get all the pistons, either, so if they need replacing you'd have to order them in.

http://www.biggred.co.uk/

http://www.nsxcb.co.uk/entry.php?1890-Brake-OH-Cooling-Sys-etc-22

I have just had Goodridge hoses installed on my NSX, along with calipers overhauled by Bigg Red. Car is much better for it. :)

[RedCarsGoFaster]

Of course, you don't need to *start* by throwing parts and money at the problem - a more intelligent approach to troubleshooting might solve it much more inexpensively!

[Pride]

Although the brake hoses on inspection passed the MOT it has been known that the internals of the hose can perish creating a restriction causing slow flow rates and even a one way valve effect in the most serious of cases.

After a bit of research I found this on YouTube although it's internally perished at it's most severe (so ignore the full locking up effect in the video) but you'll get the jist and this could be the cause of your soft pedal feel

https://m.youtube.com/watch?v=O9dh3P3eD2E&feature=youtu.be

[Nick Graves]

How are the brake discs? If they look like LP records, they can result in a softer pedal feel than flat, new ones.

I'd check it's not just a partially-seized bleed nipple - maybe a pressure-bleed might help.

I've had problems bleeding brakes after a caliper change before - for some reason (ABS module?) it's a lot harder than it once was.

[Kaz-kzukNA1]

People uses the wording 'soft' in different way.

Sometimes, it means 'spongy' feeling, just like when air was left in the brake system.
Some people felt the difference and described it as 'soft touch' when new combination of pad/disc were installed compared to the previous set.
Some people may feel it 'soft' when the brake pedal travel increased even the actual pressure generated at the end was the same. For example, the knokback effect with the fresh lip of piston seal or on track.

It 'may' help if you can clarify the meaning (spongy? longer pedal stroke? caliper just overhauled? hot? cold? speed of pedal operation? high/low pedal pressure? etc).

Anything done at the brake system before this issue happened?



As I don't know what you meant by 'soft' and I don't know the spec of your NSX (brake spec, ABS spec, brake modifications, etc), I could be talking nothing related to your 'soft' pedal feeling but some info may help others so here we go.


The flow rate will change the pressure slope but won't cause 'soft spongy' pedal feeling unless you have leakage or air in the system.

For example, if you look at the diameter of the CL Hyd hard line and the internal diameter of the rubber hose at the slave cyl, you will understand the effect on the CL pedal weight (effort) slope when using the OEM and the aftermarket hose with the different diameter. It's related to the selection of the pipe/hose diameter considering the flow speed and the amount.

Again, it won't make the pedal soft spongy but changes the pedal weight slope especially if operated the pedal really fast.
Some aftermarket hose will increase the pedal effort.
It's like suddenly, the CL pedal got really heavy.


OEM brake hose has multiple layers and unless caught something and cut through it, the flex section can last for years.
Before the flex section fails, most likely, you will start seeing tiny seepage at the crimped section at the fitting and the hose holder.

Car manufactures and brake companies are working together in this area and you can find many documents/papers with FEM models around the effect of crimping and welding of fitting, etc.
Most of the time, the rust at the crimped section will chew into the layer and with another condition started inside of the hose under/near the crimped section, it will cause seepage but you won't feel it through the pedal until it gets really bad.
It will be noticeable leakage and not just surface seepage.


Not sure of your ABS spec (classic one that makes the rattling pump noise or the latest one body) but there is minute chance of ABS issue if you have the classic one but this is like in the order of 0.0001%.
Under normal condition, the main and the classic ABS fluid are completely separated.
However, it can get mixed up if the seals failed inside the classic ABS and cause 'somewhat spongy' but with extra pedal stroke.

There is another classic ABS condition that can cause 'soft' pedal feeling (it actually feels more like 'pedal to the floor' and not spongy) but I'll leave this as this can only happen under ABS operation.


Brake vacuum booster can be tested fairly easily.
If healthy, you should be able to keep the vacuum assist even after weeks and can check the vacuum power after releasing the vacuum and starting the engine.
Without the booser assit, you will need massive pedal effort to stop the car even at walking speed.


Unless you managed to generate the air or let it enter the system (but you already bled the system), from what I saw on other NSX, I would start with the master cyl test and look for any leakage around the brake system.

If leaking at the caliper, quite often, you won't see it until it gets really serious as the seepage would be hidden by the dust boot at the piston.

Don't forget to inspect the brake pipes at the front compartment under the bonnet including the 6-way joints behind the water drain pipe next to the blower motor.

I saw many NSX with pipes scratching against the body work or against other pipes and nearly puncturing it.
The fitting at the 6-way joint can get loose without being noticed while someone working inside there.


With the master cyl failure, at the initial stage, you won't notice it if you press the pedal like when you are driving on street.

Slower or gentle operation with the car stopped would highlight it, just like sitting in the traffic jam or when you taking the car out of the garage for the first time of the day.

Most of the time, the leakage would be trapped behind the base of the master cyl because of the severe rust of the booster cover and thus, no wet marking or because the leakage is so slow that it will only show up as rust marking at the bottom side of the booster cover.

Test it with the gentle pressure applied to the brake pedal, like stopping at the traffic light when driving AT car in D range (prevent it from creeping forward).

Keep applying the same pressure as much as possible for a few minutes.

If the pedal sunk noticeably, you have master cyl failure.
Though, without actual experience with the failed unit, you may find it difficult to detect the failure with this method.....


Kaz

[L696ULO]

Hi Kaz, thank you for informative reply. I have now had time to work through most of your suggestions.

I'll start with the car's set up. The NSX has the original 1993 brake spec and original classic ABS system, it has drilled discs which were there prior to my ownership (10 years).
The front pads were changed in 2015 so well before the issue arose.

Now onto my findings. When the car is parked in garage the brake pedal remains solid however with my foot on the pedal as soon as the engine starts the brake pedal immediately 'sinks' and becomes firm again at a lower point in the stroke. The braking point remains at the bottom end of the pedal movement whilst the engine is running and once the turned off after a couple of pumps it returns to being solid almost as your foot touches the pedal.
With regards to the ABS (classic) I haven't heard the rattling pump noise for some years! The ALB warning light goes out after start up, what would trigger the light to remain on?
I've checked all the brake lines and I can't see any leaks however following your recent blogs maybe the master cylinder is the culprit. After 25 years of potentially not being touched and dirt in the foot well should it be spotless? Would it be prudent to have a new master cylinder fitted anyway? Also the ABS system hasn't been touched in my ownership so could air in the ABS accumulator be causing my brake pedal problem?

regards Richard

[goldnsx]

Now onto my findings. When the car is parked in garage the brake pedal remains solid however with my foot on the pedal as soon as the engine starts the brake pedal immediately 'sinks' and becomes firm again at a lower point in the stroke. The braking point remains at the bottom end of the pedal movement whilst the engine is running and once the turned off after a couple of pumps it returns to being solid almost as your foot touches the pedal.
That is a pretty normal behaviour. So the brake booster is working properly.

When the car passed MOT did the also put the car on a brake dyno checking if all brake are working? A bad master can be detected by pressing the brake pedal for a minute or so. If it's rock-solid and doesn't sink any more during the test-time the master is ok. But after 25 years it doesn't hurt to replace it. I guess that part of your problem is in the calipers or maybe the ABS. If the tech said that one bleeder is slow that's maybe a hint to give them a full overhaul.