Spongy Brake Pedal - 100 Series

[rooboy98]

G'day,

I am having a bit of a drama with my standard 100 series which has developed a spongy brake pedal which slowly falls to the floor when a constant pressure is applied. It doesn't feel too bad if the pedal is applied quickly (stabbed) but if reversing down a steep incline to launch the boat you now need to pump the pedal once or twice to maintain braking pressure.

When approaching round-a-bouts with the boat on, I've noticed that you don't seem to have any real brake pressure until the pedal is low.

The truck has never leaked any brake fluid.

The brakes on this truck used to be really good, however something is not well at the moment. I've owned it for 6 years now and it now has 247,000kms.

So far,
New Bendix 4WD pads and all 4 rotors machined - (2 months/3,000kms ago).
New Master Cylinder and fluid flushed - Today.
Obviously the system has been bled twice now. (by two different mechanics).

I was told by the mechanic today that the old master cylinder was indeed knackered, however the pedal doesn't really feel much better. Still spongy down low.

Any suggestions?

Cheers,
Roo.

[Blackened]

G'day

How's the slaves?
Brake assist system?

Dave

[FNQCairns]

I am with Dave just change out the slave and be done with it (re-kit and keep the old one in your 4WD trip spares box)....once at his point your only other consideration should be air.

cheers fnq

[rooboy98]

G'day

How's the slaves?
Brake assist system?

Dave

Thanks for the quick replies guys.

When you are referring to the "slaves", are you referring to the piston/seals in each of the 4 disc brake calipers?

I also asked about the brake booster assembly, but I was told that a fault in that item would only cause a hard pedal......not 100% sure if that is fully correct though.

Cheers,
Roo.

[FNQCairns]

Sorry Roo I had (yet) another short insight into what my future might hold. I answered as if you where speaking of a clutch problem:(.

IMO and answering for brakes, it will be air, sneaky air...very possibly toward the rear wheels and even pointed at any/around the load bias valve.

Sometimes it takes a bloody long time and lots of fluid done systematicly to rid all of the air.

It's not a vacuum problem.

cheers fnq

[rooboy98]

No dramas FNQ......you had me thinking though!

I tend to agree with you that there must be air still trapped in the lines somewhere. Like I said earlier it still feels pretty ordinary.

I think I'll have to take it back in and give em another opportunity to fix it.

Just an idea.......could a faulty Load Proportioning Valve cause similiar symptons?

Cheers,
Roo.

[FNQCairns]

I don't think so but I am not completely sure, at least I have never come across it, I have seen them leak.

cheers fnq

[Benny01]

you would think 2 different mechs would get air out. you can simply block off one hose at a time then check the pedal if it is no different clamp off another hose. if it goes rock hard the you have air in that line.

if the master isnt bench bled properly it can take a bit to get out but its only a 100 series. easy as.

pretty sure they are a DOT 3 master and DOT 4 will f uck them as lots of people have found out

[White Pointer]

G'day,

Throw the Bendix in the bin and possibly the rotors as well if they are showing heat marks. Restore to OE and drain and refill the braking system and bleed. Bleed again after an hour standing.

Regards,

White Pointer

[shano]

my 80 series does exactly the same thing! has also had new booster and pads! done complete fluid flush still the same!

[backlash08]

a long shot but check that the flex hoses (on front hubs and from body to rear axle) are not expanding under pressure, they can bubble with age or if vise grips have been used to block off hoses in the past. I have seen this happen before producing spongy pedal,
having said that, most likely will be air in the system, maybe even in the master cylinder, they can be hard to bleed so suggest a pressure bleed approach on these things.
push rod adjustment into the booster could also cause the feel,
I'm assuming that the truck has new fluid right through it, if not, start there. You could also disconnect the vacumn feed to the booster, block it off, start the truck and without driving it, just in neautral see if the pedal hight still drops with a constant pressure omn the pedal, remeber the pedal will get very hard without the booster but it will tell you if the booster is causing the problem.
of couse only do one thing at a time and put it back together before you start another area of diagnosis.
I suggest start with flex hose inspection and proper fluid flush

[hondaguy]

Is it diesel or petrol? If diesel did they check the vacuum pump on the back of the alternator? Is the alternator belt tight? If loose you will not have any vacuum to assist brakes.

[Noelm]

if the pedal "leaks down" under constant pressure, it is not air, it is either a very slow leak (christ knows where) or the very remote possibility of a soft hose, but pretty rare that one, did all this happen after the pad replacement?

[backlash08]

I wouldnt totally discount air in the system

[rooboy98]

OK, some good ideas here guys (keep 'em coming please).

The problem did not start after the pad change. It was more of a gradual thing.

The original pads had done a bit over 240,000 kms so I thought I'd get all pads changed and the rotors machined first. The pads have now done 3,000kms.

I wasn't surprised when that didn't fix it as I thought all along it was going to be a master cylinder problem. I then had the master cylinder changed and the fluid flushed (at a different mechanic).

Had the front calipers rebuilt (new seals) and the system bled again. This made it slightly better but the pedal still feels spongy.

I was doing some trouble shooting at home on the weekend and I clamped off the flexible brake line that runs down near the front diff before it runs out to each of the calipers. After doing this the pedal felt heaps better, so I still believe there is a problem in the front brakes...somewhere.

The system doesn't leak any fluid. I've never had to top it up ever.

I will run a check of the booster and check the alternator belt tension. Ihave had a look over the flexible lines as Backlash suggested.

I'm thinking I will also try some stainless braided lines to replace the original rubber ones on the front as I can remember the difference they made on my dirt bike years ago.

I understand the theory of a pressure bleed but I have never had it done. Can all brake workshops do it? It still feels it could be air, despite being bled by two different workshops several times.

Thanks again for the ideas. I will place the cure on here if/when I get this sorted.

Cheers,
Roo.

[Benny01]

there ya go. you have air in the fronts. u diagnosed it

[backlash08]

up to you but I would not spend money on braided brake lines, the inner of the lines is mostly the same, high quality aftermarket or genuine is all you'll ever need IF that is the problem, seriously I think you have air in there somewhere, check that all the bleed nipples are free, often they will block up, even break off making it impossable to fully bleed the calipers

[FNQCairns]

Definitely air somewhere it was simply never done properly by those who should have. Check the legals on the particular brand of braided hose you choose, you don't actually need them to 'fix' the problem though.

cheers fnq