Hello everyone,

I wanted to ask a question about my 1990 745 turbo.
The front rotors, calipers and pads were just changed and I bled the lines.
Now before the work the brake pedal intermittently fell to the floor. It still seems to happen but only when stopped at a light. If I push hard on the pedal at a stop it seems to give in and slowly go to the floor? Also after releasing the brake the engine idle drops slightly from say 800 to 600 and then rebounds? Is this a vac line problem or maybe air in the system? I tried to search the forum for an answer and came up with too many possibilities. This is very strange to me and any help would be great.

Thanks, Greg

 

Assuming you have no external signs of brake fluid leaking, I'd say you most likely have a bad master cylinder. Since the symptom existed prior to the recent brake work, it is unrelated. The variance in idle speed is a vacuum issue. Could be...1. Leaking booster to manifold hose 2. Bad check valve at booster 3. Bad booster
Is your 90 an ABS car? It was optional in 90, standard in 91 IIRC.

 

Yes I did see an ABS sensor.

 

Well, if it comes down to it and you determine it is the booster leaking, I have a booster/master cylinder unit from a 91 740T w/ ABS I'd sell cheap. Just drop me a pm some time if you need it.

 

What other signs should I be looking for to rule out if a master cylinder is failing?

 

A brake pedal should never travel to the floor. Doing so means something is not holding pressure. If there are no stains or wet spots on the inboard sides of the wheels near the brake hoses, no wet spots on the ground, and no wet stains along the frame where the steel brake line runs and the connections are dry...the only other way to lose pressure is for the seals inside the master cylinder to have worn out and allow fluid to bypass the piston seals. It's a sealed system; if no fluid is exiting, then it has to be not staying where it can get pressurized and thus the pedal creeps down. Typically, sitting at a red light, the pedal will slowly creep to the floor.Pumping the brake pedal will restore the pedal height...for a while. As the seals further deteriorate, the brakes will beging to not pump up. Eventually, you have no brakes at all.

 

Any kits out there available for a rebuild?

 

Sure...check rockauto.com Remember, a good m/c rebuild includes honing the inside bore. While you're browsing rockauto, compare the prices of a replacement m/c versus a rebuild kit.

 

Yeah I see your point $60 verses $100 or more.
The Haynes manual said honing was not necessary as long as the bore is not damaged?
They give the reason for failure as the seals and rubber parts inside the cylinder.

Well, lots to think about. I going to do #1 and #2 you mentioned above.
I'll change the fluid and bleed the whole system just in case.

It's only happened a few times since I have been driving the car.
If it happens after this I'll have to tackle the rebuild or replacement issue.
This is not the kind of thing which can be ignored.

Thanks for your help swift.

 

I have customers ask me why we will not simply replace a bad front seal in an a/c compressor. The reason is, invariably, there is something within that caused the initial failure. Any subsequent failure will be blamed on us. It's cheaper in the long run to simply replace the compressor. Likewise, it's a crap shoot to rebuild a master cylinder without honing. I did it a few years ago when I was building my 140. I was unable to locate a new or reman m/c at the time. We had a rebuild kit in stock...bore looked good so I rebuilt it. It failed this past winter. Only on cold mornings and one pump would bring the pedal up. Eventually I sourced a good m/c and swapped it out. I dislike doing the same job twice. Still, it's a common occurrence today as the quality of replacement parts continues to decline. O.E. parts can last 20-30 years. The replacement? Who knows...fuel pumps sometimes only 3-4 years. You're right though...brakes are not the place to cut corners. Good luck!

 

Ok so finally tackled this job. I have to say It was not that bad overall.
Put in a new M/C and rebuilt booster and power bleed the lines.

Everything seems ok but the brakes are still a little spongy at the top.
Is there some adjustment that needs to be done after this job?

Also after going for a test run I stomped on the brakes and all the lights came on and the car stalled. First time that has happened? Any thoughts...

Thanks all,
Greg

 

Bump

No one on the board has replaced a M/C and booster before?
What adjustment should be made to the pedal after this repair,
Does anyone know ?

Thanks Greg

 

Gregman, I haven't had to do a vacuum booster AND m/c before; but I have done some m/c's before. Good thing you got a new one; it seems like the rebuilds just don't last. My "remanufactured" m/c lasted only 6 mo. before I had the falling pedal under light pressure symptom.
I am sure there is no adjustment necessary to your pedal. I suspect the spongyness could be eliminated by rebleeding the system. I assume you "bench bled" your new m/c before putting in the car. You cannot get all the air out if you don't.
Also note that these cars have 8 bleeding nipples ; 3 on each side in the front and one in the rear. The order of bleeding is very important; I didn't do it right the first time I did it, with a soft pedal as a result. If you don't have a Bentley or Haynes manual, search this forum. Its in here on some other posts. Good luck.

 

No I did not bench bleed the system.

I thought because of the power bleeder that it would not matter?
Guess it does.

Also my car has 5 nipples to bleed from.
1 on the M/C itself and one on each caliper.

So I should try and bleed again to get rid of the air?

After starting the car the pedal falls about 2 inches.
And it starts to engage slowly at the top and then grabs well as you depress.

 

The reason for bench bleeding the m/c is the sheer volume of air trapped in one at installation. Just makes bleeding the system faster and surer. I've never owned or extensively driven an ABS Volvo, so really don't know what a correct pedal should feel like. Rebleeding the system could not hurt. And, as mentioned, there's nothing else to adjust. The pedal falls as the booster gains power from vacuum; that's normal. The car dying after a full on panic style stop is not unusual but if you just punched them hard for a moment and let off, it's possible there is one or more vac lines leaking a bit.