Brake Problem

[collins_dan]

Just rebuilt the master brake cylinder on my federal S4, so its the tandem/dual circuit variety. The brakes are really odd. One minute they will feel great. Firm and engaging almost immediately. Give it a couple of minutes of not using them, and no brakes until just before you hit the floor, or the car in front! Pump them a bunch of times and they come back. It's got to be air in the system. When I was bleeding the brakes, it never felt like the cylinder wasn't coming all the way back out of the bore, so I'm thinking its air trapped in the master cylinder. I loosened each of the brake lines coming out of the master cylinder, one at a time, pumped brake until fluid came out. Since they don't have bleeder, its hard to be precise doing this. Any ideas what it might be? Am I on the right track? Suggestions? I want to get back to driving. Thanks. Dan

[rgh0]

Hi dan

Does it only happen when driving and not when the car is stnading still? - if so check the hub bearing end float and disk run out as you may have a pad knock off problem.

Do you have brake boosters - if so thry disconnecting that from its vacuum and see if that makes a difference.

cheers
Rohan

[collins_dan]

It doesn't seem to happen standing still. How would I check "the hub bearing end float and disk run out as you may have a pad knock off problem"? Thanks, Dan

[collins_dan]

I just read the archives. I don't have a dial indicator. When I put the hubs back on, all I did was tightening the end nut to 5 lbs. ft., then backed off slightly to get cotter pin in. I did notice that the wheels felt sloppy when driving. Thanks. Dan

[collins_dan]

No brake boosters.

[rgh0]

Without a dial indicator the best way to adjust the front hubs is to tighten the bearing to the specified 5 ft lbs while spinning the hub. Back off the minimum needed to install pin as you have done.

Then mount wheel and with the wheel off the ground test how much free play in bearing by grabbing the wheel top and bottom and trying to rock it- you should only be able to just feel the movement at the tire. Try tightening up to the next increment in the spit pin / castle nut, if can still feel movement repeat until movement goes away and then back of 1 increment.

Once that is done spin hub while observing brake disk versus the caliper slot it runs in - there should be no observable variation in the gap between the disk and the caliber as the disk spins. If there is then your disks probably need machining or replacement

For the rear hubs there is no adjustment on the bearings, if you can feel any movement in the hub bearing by rocking the wheel when its off the bround then the bearings need replacement, similarly check disk run out by spinning the hub and look for any variation in the gap to the caliper.

regards
Rohan

[collins_dan]

Thanks for the suggestions. All seems correct with the front hubs (didn't remove the rears); no movement up & down, some movement right & left due to worn steering ball joints, which will get done, just not right now.

Overall, the brakes feel very firm, actually better than ever (changed to ss braided hoses), but I did notice that after sitting at idle at a traffic light with the brake applied, they felt softer. They firm right up after use and don't soften again until sitting at idle with the brake applied. I'm assuming this means air in the system and more bleeding, but just not sure where the air is. I'm thinking that it might still be in the master cylinder, which was dry due to the rebuild.

What is the proper procedure for bleeding the master cylinder? Same as bleeding the calipers? Loosen connection enough to allow fluid to flow, depress brake to floor, re-tighten, let brake up fully, loosen connection, depress to floor, tighten. I would think 2 go rounds would be enough, then bleed all four calipers.

The only other thing I can think could be causing a problem is that the right front caliper is not retracting fully, and is rubbing a bit on the rotor. I tried to push it back in, but its not moving. How can I lubricate and get this moving back in more freely? I probably need to replace the pistons and seals, as neither look very good, but would prefer to save this for winter when I do the steering ball joints and gaiters.

Appreciate the advice,

Dan

[alaric]

Hi. I have the single circuit master cylinder, but think the internals are very similar. There's an o ring seal on the end of the pushrod which stops the fluid going back into the reservoir as you depress the brake pedal. If this is not seated correctly or has a little bit of dirt in it, or if the return spring is not installed properly, it could be allowing the fluid back into the reservoir instead of to the calipers. I had this problem with the clutch master cylinder, which obviously isn't quite so scary. Strikes me that if you didn't have this problem before rebuilding the master cylinder, then it's not the run out on the disks (rotors) at fault - but I'm only suggesting stuff - the brake caliper does sound a tad suspicious doesn't it.

My father recently had the same symptoms with his V8 Esprit clutch master cylinder - foot to the floor sometimes but fine most of the time. Now he's got a new master cylinder not only does this not happen, but all our concerns about the clutch and gearbox have gone away; the clutch is now really firm and gearchange absolutely silky as could be. It must have been a poor non return seal causing the problem; we'd had the poor gearchange and random clutch pedal to the floor for some time and were being told it was the clutch sticking on the spline, which would have been pricey. The mechanic at the dealership didn't believe my theory - master cylinder - until his foot went to the floor with the engine idling parked outside; they'd already bled the system and changed the fluid a couple of times. An hour later, with a new master cylinder, and all was fine.

All the best.

Sean.

[rgh0]

I agree it sounds more like a leak at the non return seal back to the reservoir. I would strip and rebuild the master cylinder paying care to ensure this area totally right - i.e. seat area smooth, no dirt, rubber seal good, spring and rod that holds the non return valve working propelry.

I assume no other leaks in the systems either at the master cylinder or wheel cylinders

A sticking piston in the caliper should not give the problem you observe, but should be fixed if you cant push the piston back wnen levering it with a scredriver against the disk as it can give problems with brake balance.


cheers
Rohan

[collins_dan]

I just rebuilt the master cylinder, so all internal seals are new. Which one is the non-return seal? Mine is dual circuit system, so the reservoir is connected to the master cylinder in 2 locations, the main seal with the tipping valve and the more forward location, where the reservoir has a sort of nipple the pushes into the cylinder. I was unable to replace the o-ring seal on this nipple as they are not available. Is that the seal that you are referencing?
Thanks. Dan

[rgh0]

Hi Dan

I am not familiar with the details of how the feed from the reservoir is sealed when the pedal is pressed on the original dual circuit design ( it never came to Australia). The single circuit design had the port from the reservoir at the end of the master cylinder and a spring load rod and cup / seal on the end of the piston covered that port. Depressing the piston pushed the cup against the end of the cylinder sealing the master cylinder supply port. The spring load rod held the seal firm while allowing the master cylinder piston to keep moving as needed as the pedal was pressed further

The dual circuit master cylinder must have an arrangement for each circuit that seals the reservoir supply when the pedal is pressed so pressure is built in the master cylinder and fluid does not just flow back to the reservoir instead. You need to identify that arrangement and ensure it working. The O rings you refer to sound like those that seal the plastic reservoir to the body and are not the non-return seal arrangement I am talking about

cheers
Rohan

[collins_dan]

I've attached two images that show how the dual circuit operates. To look at the images, it appears that there are two possible places the tipping valve might not be properly sealing or the seal at the end might not be properly sealing. Could air trapped in the master cylinder be causing this problem? All the seals are new and everything was carefully checked and operating properly prior to re-installing the master cylinder. Thanks. Dan

[collins_dan]

I had my son pump the brakes and there was no movement in the level of the fluid in the master cylinder, so I don't think it is a return valve problem. I did discover a leak in the connection between the left front line and hose. I've now tightened that up and driven the car. I thought I had it solved, but then after going over a bump to get into the garage, the brakes felt soft. I'm thinking there is still some are in the system, and it must have moved when I hit the bump. What do you think? Do you still think I have a return valve problem, or simply air in the system? Do you think I should bleed the master cylinder as well as all calipers? If so, what's the best way to bleed the master cylinder? Thanks. Dan

[patrics]

Hi Dan,

I think Rohan and Sean are right about the master cylinder being the problem.
Typical centre valve problems tend to be that they feel okay when driving normally but when applying the brakes at low speed with much slower applications the pedal sinks.

Statically try pushing the pedal slowly and see if it sinks then try a faster application and the pedal will probably feel okay.

As Rohan has said strip the cylinder back down and check for debris on the sealing face of the centre valve.
Did you not replace the tipping valve seal?

If you want a how it works / service info I will scan it for you.

The CV design cylinders are not great at sealing with very slow applications but they tend to give plenty of warning when they need rebuilding.

If you think the calipers need rebuilding then get on with it ? doesn?t take long to do.

Regards
Steve

[collins_dan]

I did put a new tipping valve in, that's the frustrating part. Cleaned it all like crazy... ugh.