Hard braking on 3.0CS

[taylorcom]

I have an issue maybe someone can help with: in hot weather with stop-and-go traffic, my 1972 BMW 3.0CS gets harder to brake. It feels like the brake booster has stopped working. The brakes work fine, they just take a lot more leg-power.

I've checked the vacuum line to the booster, and the one-way valve, and they seem intact and clear.

Unfortunately, the brake booster is no longer available for this car as a new part.

I saw a suggestion somewhere that changing the brake fluid might help, and though I haven't changed the fluid on my CS in a while, I have doubts as to whether that would have any effect.

Does anyone have any suggestions on this??

Many, many thanks.

 

[jmackro]

I doubt the fluid is the problem, if the symptom is a lack of power assist. If the fluid is really old, and you are getting your brakes really hot, you can boil the fluid in the calipers - but the symptom of that is a spongy, not hard, pedal.

Brake boosters do die. Usually it isn't temperature related, but... There are shops that rebuild them. Look in your local Yellow pages, talk with the old timers in area repair shops, network with local automotive hobby folks, etc. A shop in the south LA area rebuilt the ATE booster for my Alfa Romeo a few years ago; the CS booster is similar, so it should be doable.

 

[bengal taiga]

I have an issue maybe someone can help with: in hot weather with stop-and-go traffic, my 1972 BMW 3.0CS gets harder to brake. It feels like the brake booster has stopped working. The brakes work fine, they just take a lot more leg-power. I've checked the vacuum line to the booster, and the one-way valve, and they seem intact and clear. Unfortunately, the brake booster is no longer available for this car as a new part. I saw a suggestion somewhere that changing the brake fluid might help, and though I haven't changed the fluid on my CS in a while, I have doubts as to whether that would have any effect.

Does anyone have any suggestions on this?? Many, many thanks.

Your post doesn't couldn't get much vaguer and raises more questions than it answers. Since your coupe's brake system is so similar to many other cars on the road, your question seems hardly coupe specific rather, it is just a common braking question about any hydraulically activated braking system.

You never describe the condition of your coupe and what (if any) maintenance has been performed. You have a 30+ year old car, have the flexible brake hoses ever been replaced? Since you admit not having changed the fluid, does that mean you don't know if it is years or even decades old or even the correct type? How about other parts of the braking system? What is the shape are the calipers, rotors and friction materials? Tires? Air Pressure?

Although I have never tried it, I would bet mineral oil would permit the braking system to function, but not well and certainly not well as soon as the system heats up. Hint: There is a reason why the better brake fluids have higher boiling points.

Before you fault the booster, have you tested it to see if it is holding vacuum? You checked vacuum lines, but you never explain that the booster is leaking. This is simple procedure that has probably been addressed in this and other forums many times.

Good luck.

 

[taylorcom]

Dear bengal taiga,

Thanks for your thoughtful reply.

Could you please tell me how I can check to tell if the booster is leaking? I couldn't find the procedure anywhere.

Thanks.

 

[bengal taiga]

Brake Booster

Could you please tell me how I can check to tell if the booster is leaking? I couldn't find the procedure anywhere.
Thanks.

I can think of several means to test your vacuum booster, the simplest is start car engine. You do not have to be moving. Depress brake pedal. Notice how the pedal feels. Now shut off engine. Depress brake pedal a couple of times and you should notice the pedal feels much harder than before and becomes increasingly harder. This is because, with the engine off, you have now cut off the vacuum supplied by running engine's manifold vacuum pressure. If there is no difference felt, you likely have a vacuum leak (your booster or the one way check valve in the line to the booster are not holding a vacuum) OR you have one hell of a linkage problem and sticking calipers/master cylinder.

For a different approach here is a something pulled from a VW site.

"Hook up the vac gauge. Start the engine. Note the reading. This is your "base" reading.

Stop the engine, disconnect the booster vacuum line and cap the port on the manifold. Start the engine, look at the gauge. If this reading is substantially higher than the base, you have a leak in the system, probably in the lines leading to the booster.

Stop the engine, remove and test the one-way valve (should be able to blow into it one way but not the other). Air should only pass in the direction indicated by the arrow printed on the top of the valve (make sure to reinstall it in the correct direction).

Reconnect the one way valve and vacuum line to the manifold and start the engine. Watch the vac gauge as a friend depresses the brake pedal. It should dip as the pedal is depressed and return to the base reading. If it drops slowly, but returns, you have a blockage/collapse in the vac line. If it falls and does not return, you have a torn diaphram in the booster. Especially indicative of a torn diaphragm is a hissing when the pedal is depressed. " http://www.type2.com/library/brakes/vacboo.htm

Here is another version of what I described above: http://forums.carcraft.com/70/6464225/automotive-brakes/testing-brake-booster/index.html

I am not the most intuitive guy on the planet, but I suspect your problem is not your brake booster. If it was leaking, you would have a very hard brake pedal at all times AND a vacuum leak usually results in a rough idle or rough running. As noted before, brake fluid, sticky calipers, old flexible brake hoses and worn or compromised brake pads seem more likely culprits.

Good luck

 

[sfdon]

A hand held manual vacuum pump/gauge is a must have tool for anyone that is a DIY mechanic. 50 bucks or so anywhere good tools are sold.
Connect it via a tee to your booster line after the check valve and you can monitor the booster in situ regardless of temp. Will also help set air/fuel ratio, timing, check engine valves and change brake fluid and check any vacuum actuator with a diaphram.

 

[RonP]

SSF has this booster new in their Bay Area inventory. I had asked them to ship me one for the 2800CS and they sent one for a 3.0CS. I sent it back last Tuesday. BTW, I had the same problem you are experiencing with my car. Carl Nelson sent me one of only two boosters he had (new).

 

[taylorcom]

ronp,

this is interesting. thanks. please tell me if the new booster solved the problem on your 3.0 CS.

btw, I also have a '70 2800 CS.

steve