How does air keep getting in my brake lines?

[Tyler]

Brakes go soft 24 hours after bleeding. All major components either new or recently rebuilt. Lots of air is entering the system somewhere between the resevoir and the top brake booster ('72 RHD dual circuit), demonstrated when bleeding the booster using pressure at the resevoir. The other booster bleeds no air. There are no fluid leaks anywhere.

Funny thing is, the brakes temporarily go firm when engine vacuum on the boosters is high (ie, coasting downhill in first gear).

Can anyone suggest how to diagnose where the air is getting in, or why high vacuum would momentarily fix the problem?

Thanks!
Tyler
'72 CS RHD Baikal
London

 

[vraned]

Tyler,
From your description, it sounds like the rear seal of your master cylinder is bad. High vacuum from the booster is being communicated directly through this seal to your rear brake circuit: and probably drawing some brake fluid into your booster. When the booster returns to lower vacuum conditions, air is then leaking back into the rear circuit of the master cylinder, past this seal, to restore pressure equibrium. So, you have air bubbles/brake fluid shuttling back and forth across this seal, depending on the manifold vacuum. Also, the brake fluid will cause the inside of your booster to rust.

 

[Malc]

Being RHD you have to deal with the Twin remote Servos, which can be a pain in the bum :roll:

If your not loosing fluid then you probably have air trapped in the system somewhere, especially if you "pump" the pedal it then goes "hard".


Anyway Suggest you try and check the following:

1.. Check one way valve in hose from manifold to servos is in the right way round and working.
2.. No splits or leaks in the vacuum hose.
3.. Use a pressure bleeder, Guyson do a reasonable one
4.. Always bleed the two servos first.
5.. Then do the back calipers, then the RHS front then the left front
6.. On the fronts do the inside, outside then the top (Correct me if I am wrong please)
7.. The bit that always gets forgotten because they are such a pain to get at are the remote servo air filters, on the bottom difficult to get to. If they get clogged the diaphrams cannot "breath" properly
8.. THe master cylinder can start to go, check around in the footwell for fluid seepage.
9.. Remote servos fail, as stated and start to fill with fluid

You can get "new" servos but they are very expensive. However there are exchange ones available (Jaymic et al) and there is a company that will resleeve them in stainless steel, can't remember the name, but they often advertise in magazines such as Practical Classics.
Remotes are the same as a 2002Tii.

HTH
Malc

PS Added after thinking about it for a while....
10.. Check the hose from the reservior into the master cylinder, plus the plastic dooby the fits in the "bung" on top
11.. Pump the pedal at the same time as having a pressure bleeder on!
12.. Don't use a lot of pressure in the bleeder - 10 - 15psi is enough
13.. Is the master cylinder new?

If the pedal goes to the floor after leaving the car overnight, but pumps up after a couple of goes, keep you foot on the pedal and start the car, does it go down further? - if so I would suspect a valcuum leak somewhere. Same after you have run the engine, turn it off and pump the pedal, does the feel change? - leave it over night - different again?

Sorry to keep chucking this stuff out but it's how the remains of my brain works!

 

[Tyler]

Malc,
Thank you so much for your continued insight into my long running brake problem. We twin servo folks are definitely in the minority here, and I appreciate your time.

There is definitely air getting in somewhere. I have pressure bled it a dozen times, in the proper order, but the air keeps getting in! I suspect its somewhere between the resevoir and servos, as alot of air gets bled out immediately when I bleed the top servo. I use about a quarter of the resevoir before getting a steady zero-bubble flow.

I have to pump the pedal alot to get it hard, and even then it doesn't really firm up. The only times it is truly firm is with 24 hours of a power bleed, or occasionally when engine vacuum is very high (like when slowing down rapidly by downshifting).

MC is 4 years old, plus had a rebuild since then. I'm sure the previous owner had the same problem, as he spent a fortune on new brake components during the 3-4 year period before I took over (MC, servos, calipers, flex lines).

The bung on top of the MC is a great suggestion, as it looks a bit loose. So obvious, perhaps that's why I've avoided checking it so far...

Regards,
Tyler

 

[Malc]

Tyler,
If the braking system is relatively new then check all the engine bay pipe connections are nipped up tight. not kinked (causes fractures) and the ends of the servo's where the "splitters" are there should be crush washers present.

The system can only pull air in as the system "relaxs" so by the sound of things it's somewhere in the plumbing associated with the MC to servos area.
Assume your reservor is a common one for the clutch as well?

What else while I am at it, make sure that the piston the the MC is returning all the way

I had such a fight with my 2002Tii which is basically the same RHD setup that this time when rebuilding the dear old bucket I am going with the brake mechanism and hydraulic servo of my now dead E28

Good luck and if I get down to the smoke I might just bring my best garage clothes!

Cheers
Malc

 

[gazzol]

Hi Tyler, I agree with vraned on this one I think that the problem lies in the hydraulic cylinder on the servo. Think about it, if air is entering the system between the master cylinder and the servo then this is where the problem has to lie. This would also explain why you can't find any traces of the leak because the fluid is running into the vacuum chamber and not on the floor of your garage. The reason the brakes are good with high vacuum is because the air that has entered the cylinder is being sucked back out by the vacuum being replaced by brake fluid.
Try taking the servo off and shaking it to see if you can hear the brake fluid inside.

 

[Malc]

Gazzol,

Your right.....Sometimes the servos fill up with so much fluid it gets sucked into the engine - horrible smelly white smoke.I have seen it
I should of thought that the fluid has to go somewhere and if it's not on the floor...... Mind you it doesn't need a lot of air to make the system feel like pooh so hopefully Tyler doesn't have a major repair bill on his hands

BMW's bodge for right hand drive certainly gives folk headaches.

Malc

 

[Tyler]

Thanks guys. I will check the servo to see if its full of fluid. Something tells me its not, since the fluid level in the resevoir does not go down. Hmmm...on further thought, I suppose that as long as the lost fluid is replaced by an equal volume of air, then the level would not go down either! Is there any way to check the servo for fluid without removing it and all the hard lines?

Regards,
Tyler

 

[Malc]

'Fraid not
Malc