Brake problem

[oscar]

On the 280S I noticed the brake pedal was going to the floor some weeks after the last race.  I've only just gotten round to looking into the issue. 

Firstly, there's no leaks after a couple of new hoses and all calipers resealed.  When the engine's off I can pump the brakes to get the pedal stiff.  Then when the engine starts the pedal will move to the firewall.  Foot now off the brakes, I rev it a bit then apply the brakes and although there's resistance the pedal still sinks all the way to the floor with firm pressure. 

I can't work out what it is, I think it's the mastercylinder but I'm not sure.  Any thoughts?

Also, is it hard to change mastercylinders, if that's the problem?  Any pointers or dangers to look out for.

EDIT: I forgot to add that I've bled the brakes and there's no change.

[TJ 450]

Changing master cylinders is pretty straightforward, there is just a square profile o-ring between the master cylinder and the booster as well as the two grommets for the reservoir. I'd say the most difficult and annoying part is ensuring that no brake fluid is spilt over the paintwork.

Tim

[13B]

Pump the brake pedal with the engine off 5 or 10 times to deplete the brake booster vac reservoir.  Hold your foot on the brake and start the engine, you should feel the brake pedal fall away as the vacuum assistance kicks in.  If so we know the brake booster is OK.

Some cars, in addition to requiring bleeding at all 4 wheels, also need the master cyl bled up where the hydraulic hoses fit.  Get someone to apply the brakes and loosen the hoses and see if any air (it won't be much) comes out before the fluid does.  Bear in mind this process is messy.

However given your descripition is sounding more and more like the master cyl, better put the one from the brown car on and see how you go.

I.

[oscar]

Thanks guys.  On your first point 13b, it does seem the booster is ok.  I checked the brown cars pedal which is solid.  It's also got the stepped master cylinder which is compatible, so I've just taken that out and am just about to take povo's out.  You're right TJ, it's pretty straight forward.  I might try bleed the master cylinder as well, just to make sure it's cmopletely full.  Thanks agian. i'll let ya's know how I go.

[pez]

Check the one way vacuum valve from the engine to the vacuum booster.

[oscar]

Hadn't thought about the one way valve but it's ok.  When I pulled povo's cylinder off you could hear the vacuum started to leak.  I forgot to press the brake pedal a few times to use up the vacuum and it was standing for a while whilst I removed the other master cylinder. 

But now I've put the new cylinder on and used a litre to bleed the system, the brakes are tight when you pump them but as soon as you start the pedal will go to the floor.  Even when I've turned the engine off and used the vacuum up, you can feel the pedal sink after putting a fair amount of pressure on the pedal.  Wont go to the floor, it's better than the other master cylinder but not good enough.  Might have to get a rebuilt one, not sure I want to try a rebuild yet.

[13B]

Are all your brake pads nice and thick?  While this is not much of a solution, I would try putting on some calipers from your other parts cars and see if that help isolate where the leak or compression is happening.

About all we have established so far is that the brake booster is functioning properly.  Aim for a rock-hard pedal without the engine running first.  I know you kitted all 4 calipers but try others just to rule out any problems with them.

As you know I re-kitted all 4 calipers on whitey, installed new pads and hoses, gravity drained all the oil fluid out before re-fitting the calipers, refill and bled the 4 wheels only, and the system came up like new.  Didn't touch the master cyl or brake booster at all. 

The feel of the system was a bit deadwood and the pedal would sink slowly down due to the leaking LH rear caliper.  After this change it felt delightfull responsive and confident. 

I.

[oscar]

All pads are thick, changed before Easter but you know I aint keen on going round and exchanging calipers,  but I know where you're coming from, isolating the problem.  So I went and murdered three brake line ends, crimped the cut end and filled the with solder then put those three blank lines in the master cylinder.  Pedal was rock hard with or without engine running.  Actually, no difference whatsoever when the engine's running no matter how hard I push. No sinking at all.  I think we can say now that the master cylinder is good (the old one is probably ok too though I've started pullig it apart) plus the booster as we've said is good,  I'm going to test one line at a time now to see if I can isolate the problem circuit.  I hope it's not calipers, it's got to be in the hoses or my bleeding was crap.

Here's some of those blanked lines and in use.

[TJ 450]

FWIW, I have the same problem now and I think it was due to my poor attempt at bleeding the system, all the calipers have been rebuilt. Coincidentally, I have the same early type master cylinder.

Tim

[oscar]

Which one do you call early BTW, stepped as shown or flat barelled.  I've got some pics which I'll put up soon but I always thought the type pictured above is a later style but there is a third version or at least the reservoir is different.  The Big Triangle reservoir.  not sure if it's master cylinder is different again or what. 

Well after that little test and considering there is a tiny amount in the system, I did one line at a time, replacing the bungs after each test so i could confirm the feel when all bungs were put in.  It was rock hard at all times when all ports were plugged.  What was obvious though was both front lines have a lot of give.  It appears to me the new front flexible hoses are shite.  The don't look like they're expanding but do move a little.  Well, I better hold off writing these things off until I put some rdigy didge ones on which are in the boot of the 350.  I'll be back.  BTW, these front lines I have installed at the moment I bought in a complete set for $25US from Argentina a year or so ago.    I can feel your scorn men! 

[TJ 450]

The one pictured is the early version of the stepped type, the one with the traingular reservoir is the late version also used on W126s. Of course, the 6.9s only came with the two versions of the stepped type... I was forgetting about the flat-barreled version.

Tim

[13B]

So with the fronts plugged and the rears "on-line" is the pedal still hard and not falling?

[oscar]

Thanks for confirming that TJ.

So with the fronts plugged and the rears "on-line" is the pedal still hard and not falling?

Nearly correct so far as it's nearly ideal like that but not quite.  The pedal moves in a little bit more than when all were plugged but stiffens up quickly and doesn't drop away when the engine's started.   NB the rears still have old rubber lines on due to one of the nuts being siezed and damaged beyond repair.  So there is probably some flex in those rear lines but nothing like the front.  They don't need to be changed in a hurry IMO but I have to replace the steel line to the left rear caliper before I do both rear rubber lines at the same time.   Wish I bought those braided ones PB got a while back.   

I've nearly got it all at test stage again, just had to stop for dinner and stuff. Fingers crossed it's just hoses.

[TJ 450]

NB the rears still have old rubber lines on due to one of the nuts being siezed and damaged beyond repair.  

That is also the same problem I'm faced with... I'm going to have to leave them until after the rego inspection then get new lines made up.

Tim

[oscar]

Well I'm still at a loss.  Put new hoses on the front and did a full bleed of the system.  It feels pretty much the same as it did this morning with the ability for the pedal to just about hit the floor when the engine's going.  I plugged the rear outlet again so just the fronts were on and I still get significant sinkage of the pedal when the engine's running.  Might have to double check those pads but I really can't think what's going on.

EDIT:  BTW, I'm using new Castrol Super Dot4, forgot to mention.  Same stuff I've used in this car since I got it.