80 series spongy brakes

Submitted: Sunday, Jun 27, 2004 at 11:37
ThreadID: 14176 Views:12396 Replies:7 FollowUps:4
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Hello all

I have the same problem as reported in post 6461, namely, hard brake pedal with engine off and spongy pedal with engine on. The pedal slowly sinks most of the way to the floor but can be brought up by pumping. Vehicle is a '93 GXL 80 series turbo diesel with ABS. Symptoms sound to me like air in the system or master cylinder failure.

So far in the last week I've had the master cylinder re-sleeved, all four rotors and pads replaced, calipers lubricated and the entire system pressure flushed through all four calipers and the proportioning valve. All done professionally by reputable brake service. No change. Turning ABS off makes no difference. There are no hydraulic leaks and all flexible hoses appear normal with no evidence of swelling during operation. The prevailing attitude seems to be keep replacing componentry until the problem goes away. Firstly, I'm still not convinced that it's not the Master cylinder or air in the system, but more importantly, surely there must be some way of progressively isolating components in the brake system to identify the faulty part - I'm an electrical engineer and that's the way I'd approach an electrical fault. Seems to me that one should be able to disconnent brake lines from the master cylinder and plug outlets with bleed screws and test. Similarly, clamp wheel hoses to isolate calipers (hmmm ... maybe that's not such a good idea unless hoses are then replaced). No one I've talked to seems interested in that approach. I've had a couple of comments about the ABS modulator but shudder to think how much that would cost to replace.

Firstly, can anyone recommend a brake service with a more intelligent approach to diagnosing problems than just replacing more and more components? I live in Canberra but am willing to go to Sydney or Melbourne - I have a 2 month trip planned for September and have to get this problem licked before then.

Secondly, I'd be grateful for any advice from those with experience of this problem, particulalry with this vehicle.

Thanks in advance for any help

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Reply By: Mrs Diamond - Sunday, Jun 27, 2004 at 13:04

Sunday, Jun 27, 2004 at 13:04
sounds like you have covered everything.
try vise gripping each individual caliper hose one at a time and see if it gets better when example you clamp of right rear pedal gets hard you will then know at least what part of the system is playing up.
but it does sound to me like master cylinder
AnswerID: 65327

Reply By: Member Eric - Sunday, Jun 27, 2004 at 14:16

Sunday, Jun 27, 2004 at 14:16
I am with Jim . Sound like pedal retension to me . Pads can make a huge diffrence in cruisers . You haven't overhauled your calipers I see . As jim said , make sure you clamp off each caliper to see if it helps the pedal , if not , then you have a stuffed master . It was overhauled I know , but dosn't say they didnt stuff up a seal during assembly . Also try gravety bleeding instead of pressure bleeding .
AnswerID: 65340

Follow Up By: MikeV - Tuesday, Jun 29, 2004 at 18:02

Tuesday, Jun 29, 2004 at 18:02
Thanks for the comments - I agree that everythink still points to the master cylinder and that's where I'll be concentrating my efforts. Calipers seem fine - all the moving parts move freely when they're supposed to, and there are no hydraulic leaks. More testing tomorrow.


FollowupID: 326610

Reply By: Bilbo - Sunday, Jun 27, 2004 at 16:09

Sunday, Jun 27, 2004 at 16:09
Clamping each brake hose is a good idea and won't do any harm to the hose as long as you don't crush it too tight - it's a "touchy feely" thing. I've done it for years on various vehicles when changing wheel cylinders. Nothing's ever burst yet. Most mechanics do it.

Alternatively, buy some suitable brake pipe fittings with a short length of brake pipe flared into 'em and seal off the open end - bend it over, flatten it with a hammer - add solder if you want to. Screw in this temporary fitting where required for each test. The temp fitting won't be expensive and any good workshop or supplier will do 'em for you.

Whilst "individual isolation" is one approach that you could take, I don't think the problem lies there. It sounds more like the Master Cylinder. Did you fit all new seals AND Master Cylinder check valve? I've known new check valves be faulty and this allows the brake system system hydraulic pressure (i.e. fluid) to leak slowly back into the master cylinder - that's what the check valve is designeed to prevent. It stops it brake fluid that is under pressure from flowing back into the master cylinder bore and collapsing the cylinders rubbers thus allowing the brake fluid to flow back into the master cylinder's reservoir.

Another oddity, I've experienced is the method or "rotation" of the brake bleeding. There's often a peculiar and particlar way to bleed brakes on some models. The best way to start is to do the wheel that is furthest from the master cylinder and then progressively bleed each wheel individually until you get to the one closest to the master cylinder. e.g If your master cylinder is on the driver's side firewall, then you would bleed the passenge side rear wheel first, followed by driver's side rear, followed by passenge side front and finally driver's side front.

The reason for this, particularly if you've just flushed and refilled the whole system, is that air hangs around in odd "corners" of the system. By doing the longest brake pipe run first, you are doing everything possible to flush out these air pockets. I learned this trick from an old mechnanic, when as an apprentice, I couldn't "get a pedal" on a brake system that I'd just fixed.

Also, check the brake booster vacuum diaphragm - big, round thing at the back of master cylinder. If it's got a slight leak, gone "snotty" or small hole in it, it can allow the pressure in the hydraulic system to back flow as if the check valve was leaking - see above. The diaphragms are designed to have atmospheric pressure on one side and a vacuum on the other - supplied by the inlet manifold or a vacuum pump if it's a diesel . If the diaphragm is leaking it allows the pressure in the brake sysetm to overcome the pressure you've applied with yer foot - fluid then leaks back into the master cylinder as the pedal slowly falls..

AnswerID: 65360

Follow Up By: MikeV - Tuesday, Jun 29, 2004 at 17:51

Tuesday, Jun 29, 2004 at 17:51
G'day Bilbo

Thanks for the long and detailed reply - I appreciate the time you took to make your comments. A couple of points:

I never knew that the Master Cylinder had a valve in it - I thought pressure was equalised by an internal compensating passage in the body of the cylinder, much like in a carburettor - do you have any more info, like where the valve is located - in the MC, or externally in the pipework? Or are we talking about the power assist check valve - that one I know about?

Vacuum is fine - 30 minutes after stopping I can disconnect the vacuum line from the booster and still get an inrush of air.

Like you, I think the problem is still the master cylinder - the Cruiser goes back tomorrow for a stripdown and check of the Master Cylinder and further testing, including brake pedal pushrod to Master Cylinder piston spacing, isolation and elimination of calipers as the problem, as per other posts, and another power flush.

Still looking ...

FollowupID: 326607

Reply By: carazy merredin wa - Sunday, Jun 27, 2004 at 16:34

Sunday, Jun 27, 2004 at 16:34
has this fault just started?, when did you notice this fault? , what were you doing when it occured? does brakes/pedal work normally while driving & brake pedal slowly sink to the floor only while stationary-i think you will find this is normal for ABS sytems. You could take the vehicle to TOYOTA dealer for expert advice-even if its only for a test drive with the foreman.

AnswerID: 65367

Follow Up By: MikeV - Tuesday, Jun 29, 2004 at 18:00

Tuesday, Jun 29, 2004 at 18:00
Hi crazy - by the way, that's just how I feel now after unsuccessfully chasing this problem for a fortnight.

Fault has been progressive - certainly it wasn't there when I bought the cruiser last September. It first became really noticable driving offroad a month or so ago while driving down some fairly steep hills - over a period pedal just sank lower and lower. Now it does that just sitting in the driveway while not moving. There's not much difference in pedal travel characteristic while driving or stationary. Also, I've added a disconnect switch so that I can disable the ABS - turning that on and off doesn't make any difference.

The guys I take the cruiser to for service are pretty well connected to Toyota and have picked their technical brains pretty much dry - everything keeps coming back to the master cylinder so I think I'll have to bite the bullet and get a new one, but at $1000 fora genuine one and $800 for an aftermarket one, I need to be convinced that I'm on the right track.

Thanks for the comments

FollowupID: 326609

Reply By: Member - Jack - Sunday, Jun 27, 2004 at 17:22

Sunday, Jun 27, 2004 at 17:22
Hi Mike:

I have '91 80 Series GXL with the same problem. I got best results by changing the brake fluid. Seems over a period of time water gets in and can turn to steam under braking.

However - I put this same question to the guy in my 4WD club who is/was a Toyota engineer and he advised me that it was common to 80 Series. He further stated that the later models had larger brakes, but then it becomes a matter of how much money one wants to throw at the problem if you want to upgrade. If you have a fax number I can fax you the reply I received.

My email is jeller@bigpond.net.au

The hurrieder I go, the behinder I get. (Lewis Carroll-Alice In Wonderland)

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AnswerID: 65378

Reply By: Baz (NSW) - Monday, Jun 28, 2004 at 20:35

Monday, Jun 28, 2004 at 20:35
I know what i'm about to say might sound strange but have you tried reverse filling, i had the same problem once, difference was it was a motorcycle, what we did was force brake fluid though the caliper to the master cylinder instead of through the master cylinder to the calipers, just another idea to think about, amazing how air can sit in a system and be hard to get out.

AnswerID: 65578

Follow Up By: MikeV - Tuesday, Jun 29, 2004 at 18:06

Tuesday, Jun 29, 2004 at 18:06
Thanks - interesting idea re the reverse flushing - I'll give that a go.

The cruiser's brake system is pretty complicated, and given that a mushy pedal seems to be a chronic characteristic, I wouldn't be at all surprised to find that there was an airpocket somewhere.

FollowupID: 326611

Reply By: Bilbo - Tuesday, Jun 29, 2004 at 23:16

Tuesday, Jun 29, 2004 at 23:16

IF IT HAS A CHECK VALVE - not all do, it's located at the end of the MC piston, inside the MC. Sometimes has one for each system. So you could have 2 on split braking system. First you have the big rubber boot seals, they do main pushing of the fluid, then you have another smaller rubber/steel check valve. You'll need to pull the guts out of the MC to see it - if it has one.


AnswerID: 65744

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