Sinking brake pedal... aaarrrggghhhh

[andyburns]

Hi guys, I am having an issue with my 67 250s break setup and would appreciate any help on offer.

I am in the final stages of restoration http://www.flickr.com/photos/andyburnsnz/ and have replaced just about everything, including a the brake booster and master cylinder with brand new factory components.

I have an issue with sinking pedal and imediately put it down to a faulty 'brand new' master cylinder. I bit the bullet and brought a new one and had it tested professinaly prior to installation. The old one 'new one' that came out showed no signs of leaking out the back seal and the booster is dry. There are definitely no leaks anywhere in the system.

With a disconnected vacumn hose I bleed the brakes up with over two litres of fluid to ensure there was absolutley no air in the system. I got a rock solid pedal with no boost assist.

With the vacum connected back up and the car running I get the sinking effect almost all the way to the floor. I can here a very distinctive 'hiss' from the booster from the inside of the cabin.

When the car is turned off the sinking continues for four of five more depressions of the pedal until the vacumn is expelled from the booster. With each press the sinking deminishes until when there is no vacumn you are back to having a firm pedal again.

The only thing I can think off is that when the booster is operational with working vacum that the force applied to the master is much much more than you could apply unassisted and that perhaps I have a bulging or swelling brake hose which is faulty only under increased pressure due to the effect of the booster.

I am driving myself crazy with the problem as my car isn't warantable like this and is at the end of a two year resto. All I want to do is drive it!! Any help or advice appreciated.

I can supply the details of the break booster and master cyclinder part numbers if required. I have wondered reading other threads if I have the correct parts.

[tinypanzer]

I dunno squat about your setup.

Does your interface between the booster and the master cylinder have a "secondary freeplay adjustment"?

Some older cars have a little nut or similar that you adjust to change the length of the push rod that connects the two. An improper adjustment here can send the pedal to the floor before the brakes kick in, or make it so that the brakes don't release even with the pedal all the way up.

That's the first thing I would check..... On my old boat, the adjustment is made to the booster, where the rod comes out. There's a little nut behind the ballhead that mates with the master cylinder. I think there's even a locknut behind it. You adjust it a small distance, re connect everything, test, lather, rinse, repeat.


Although, I do think it strange that this only happens with vacuum applied. Not sure if that fits the secondary freeplay criteria. I'd check it anyway.





-tp

[450slcguy]

Did you bench bleed the master cylinder before installing?

[t walgamuth]

Sinking pedal suggests a worn master cyl to me.

[tinypanzer]

"Sinking pedal suggests a worn master cyl to me."

Re-read the post. This is his SECOND brand NEW master cylinder.....

BTW - That car is friggin' GORGEOUS!

-tp

[Paul T]

The car running supplies vacuum, which gives the power needed. Once you let the vacuum escape (car off, pedal pressed several times,) the brakes do not have the power to create the symptoms.
Check that all of the pads are moving freely , towards the rotor and away from the rotor. I have had pads hung up on rusted locating pins, and the pads do not move, but rather distort as the pedal exerts more and more pressure, and the pedal goes near the floor before it sees full resistance.

[andyburns]

Originally Posted by 450slcguy

Did you bench bleed the master cylinder before installing?

I sent the brand new cylinder out to a pro who I assume bench checked it. I installed it dry but bleed the cylinder in place with all the brake pipes disconnected. I quickly got a cylinder of fluid and good pressure and then reconnected the system. I bleed more than two litres through the lines.

[Arthur Dalton]

What procedure did you use to bleed the system ???

[andyburns]

Originally Posted by tinypanzer

I dunno squat about your setup.

Does your interface between the booster and the master cylinder have a "secondary freeplay adjustment"?

Some older cars have a little nut or similar that you adjust to change the length of the push rod that connects the two. An improper adjustment here can send the pedal to the floor before the brakes kick in, or make it so that the brakes don't release even with the pedal all the way up.

That's the first thing I would check..... On my old boat, the adjustment is made to the booster, where the rod comes out. There's a little nut behind the ballhead that mates with the master cylinder. I think there's even a locknut behind it. You adjust it a small distance, re connect everything, test, lather, rinse, repeat.


Although, I do think it strange that this only happens with vacuum applied. Not sure if that fits the secondary freeplay criteria. I'd check it anyway.





-tp

Thanks I will have a close look at this when I pull the booster out next. I have been sort of trying to avoid it as its such a prick of a job to get out. I have another old booster so I might have a close look at this to get a feel for it. The only strange thing here is that when the booster has vacuum there seems to be no travel before the brakes start engaging. If there was a mechanical gap between the two unit I would have suspected that there would always under all circumstances be excessive sinking travel. I need to get a parts breakdown of the booster and understand the principles involved. To be quite honest its all quite black box to me which is probably the reason the problem exists. Thanks for your suggestion I will definitely check it out!

[andyburns]

Originally Posted by t walgamuth

Sinking pedal suggests a worn master cyl to me.

I know thats one reason, but as I have been through two new factory cylinders it would point towards it not being in this case. Unless I am destorying the cylinders in the bleeding process I dont think its the culpret.

There are multiple scenarios that could theoretically cause this it just a matter of comming up with procedures to eliminate each on in turn.

I think my next move will be to 'blank' of the master cylinder from everything other than the master cylinder to try and pin it down the one of the two units and exlude the rest of the braking system. Does anyone know a good method of blocking the holes in the master where the brake pipe flanges normally go. I dont just want to screw a bolt down in the event I damage the flange. Once again any advice on this much appreicated

[andyburns]

Originally Posted by tinypanzer

"Sinking pedal suggests a worn master cyl to me."

Re-read the post. This is his SECOND brand NEW master cylinder.....

BTW - That car is friggin' GORGEOUS!

-tp

Thanks for the comments on the car. I am actually a bmw man but the car was recently gifted to me from my grandfathers estate. So its been in the family for a long time and I thought it was well worth presevering. The old boy picked it up from the factory in Stutgard as a tourist delivery, drove it through europe and then put it back on a slow boat to New Zealand.

[andyburns]

Originally Posted by Paul T

The car running supplies vacuum, which gives the power needed. Once you let the vacuum escape (car off, pedal pressed several times,) the brakes do not have the power to create the symptoms.
Check that all of the pads are moving freely , towards the rotor and away from the rotor. I have had pads hung up on rusted locating pins, and the pads do not move, but rather distort as the pedal exerts more and more pressure, and the pedal goes near the floor before it sees full resistance.

Hey thanks for that Paul, thats another scenario I hadn't thought off. I have had the car up on a jack and checked that the wheels arn't binding with the brakes but never considered that a pad might be held mechanicall too far from the disk. Good thinking. I will add this to my list of things to check for. The car had been sitting for upward of five years prior to restoration so its more than possible that something has rusted up. When this happened to you did your car pull toward one side?

[andyburns]

Originally Posted by Arthur Dalton

What procedure did you use to bleed the system ???

Hi Arthur, I first ensured that there was fluid in the master by bleeding it connected to the booster but disconnected from the rest of the brake system. Then I bleed out all the air I could from the connection at the brake booster pipe flanges. Then starting with the front slaves I bleed the system with a helper gently on the pedal and myself on the spanners. I have used this technique on many many cars over the years and never had any problems. Once again I had no issues getting 'airless' fluid at good volume out any of the bleed nipples. I have replaced both the rear hoses with brand new mercedes OEM items and the front two look in excellent shape so I didnt' bother.

[Arthur Dalton]

The best way to bleed is to apply pressure to the res w/o moving the pedal... 5 psi is fine. I put a long, clear plastic line on each bleeder , with the end into a can to catch the fluid..the reason for this is if there is ANY air left in each line/caliper, it will show as an air bubble in the plastic line as the fluid exits each wheel. And make sure you use plenty of fluid, specially on the rears b/c when changing the Master, you are allowing air to enter way up front and it has to travel all the way to the rears ..I see guys say they bled the rears and no air , not knowing the offending air is only 1/2 way down the long line to the rears as they never let it reach the rear..it is still in the line. All the fluid that was originally in the line has to be expelled with new fluid for the air to get out each bleeder, so the rears take a lot of fluid to clear.
Hose constrictions are also possibles..

Here is a trick I would use , with the help of an assistant.
Jack each tire and hand turn the wheel as helper applies/releases brakes w/engine off. It should take your assistant very little foot pressure to lock each wheel and each wheel should release right away as he releases the pedal. Any that don't pass both test are the suspect. If the wheel does not lock, open the bleeder and see if fluid come out there..if it does not release , then open the bleeder w/o pedal pressure and if it then releases, the hose is constricted. If it still locks when bleeder is open, then the caliper piston or wheel cylinder is sticking.
Cars that sit for long periods have mechanical sticking/binding problems all over the place and each wheel has to be addressed.

Just remember this important fact.
The booster has nothing to do with the fluid system..it only AIDS the pedal in exerting more mechanical force...the rod from the pedal to the master is one piece and the vac diaphragm only helps move that rod....applying more pressure to the system...if the pedal goes down w/booster, it is b/c that additional force on the system is causing the fault in the system to create the condition. whereas your foot prssure is not adequate to bring on that condtion....
Check each wheel , one at a time and re-bleed the whole system with a pressure bleeder would be my test to locating the problem.....

[andyburns]

Originally Posted by Arthur Dalton

The best way to bleed is to apply pressure to the res w/o moving the pedal... 5 psi is fine. I put a long, clear plastic line on each bleeder , with the end into a can to catch the fluid..the reason for this is if there is ANY air left in each line/caliper, it will show as an air bubble in the plastic line as the fluid exits each wheel. And make sure you use plenty of fluid, specially on the rears b/c when changing the Master, you are allowing air to enter way up front and it has to travel all the way to the rears ..I see guys say they bled the rears and no air , not knowing the offending air is only 1/2 way down the long line to the rears as they never let it reach the rear..it is still in the line. All the fluid that was originally in the line has to be expelled with new fluid for the air to get out each bleeder, so the rears take a lot of fluid to clear.
Hose constrictions are also possibles..

Here is a trick I would use , with the help of an assistant.
Jack each tire and hand turn the wheel as helper applies/releases brakes w/engine off. It should take your assistant very little foot pressure to lock each wheel and each wheel should release right away as he releases the pedal. Any that don't pass both test are the suspect. If the wheel does not lock, open the bleeder and see if fluid come out there..if it does not release , then open the bleeder w/o pedal pressure and if it then releases, the hose is constricted. If it still locks when bleeder is open, then the caliper piston or wheel cylinder is sticking.
Cars that sit for long periods have mechanical sticking/binding problems all over the place and each wheel has to be addressed.

Just remember this important fact.
The booster has nothing to do with the fluid system..it only AIDS the pedal in exerting more mechanical force...the rod from the pedal to the master is one piece and the vac diaphragm only helps move that rod....applying more pressure to the system...if the pedal goes down w/booster, it is b/c that additional force on the system is causing the fault in the system to create the condition. whereas your foot prssure is not adequate to bring on that condtion....
Check each wheel , one at a time and re-bleed the whole system with a pressure bleeder would be my test to locating the problem.....

Thanks that sounds like some solid advice. I will go back and take another look at the bleeding again as well. I did put more than twice teh fluid through the system than the maual stated it contained in total so unless the air is being introduced somewhere I think I have covered this on off. I did use a clear bleed pipe with a one way valve during the bleeding so I am certain that not even pin prick air bubbles were left. 'Nothing but fluid' Is there anyway you know of pressure bleeding with expensive equiptment.