Hard braking on 3.0 CS, part 2

taylorcom

taylorcom

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Hi all,

I've posted on this in the past, but want to add new info. The issue is that in hot weather after lots of stop-and-go traffic, my 1972 BMW 3.0CS gets hard to brake. The brakes work fine, they just take a lot more leg-power.

My guess is the brake booster has stopped working in these conditions (always warm weather, and lots of stop-and-go driving).

In cool weather, however, the brakes work great, very sensitive to the slightest toe-tap on the pedal.

Here's what I've done to the braking system in recent years:

-- replaced both front calipers and brake lines, using rebuilt calipers.
-- replaced one of the rear calipers w/ a new one (not sure if line was replaced).
-- checked the vacuum line to the booster, and the one-way valve, and they both seem intact and clear.

So the car has fairly recent front calipers and lines, one very old rear caliper, and probably 2 very old rear brake lines.

My question is: could an old rear caliper and old rear brake lines cause the hard braking in hot weather?

Or is the culprit more likely the brake booster?

If it's the booster, where can I get a replacement?

Thanks,

Steve
 
Do you occasionally blip the throttle? Engine vac at idle is pretty poor, and the brake booster functions off engine vac. If, after braking repeatedly without revving braking gets difficult, it may just be that the engine isn't producing enough vac to keep up with the requirements of your braking.
 
Steve, no.

First. Most of your stopping is done by the front brakes (that's why they're bigger in many ways than the rears).

Second. While it's possible you have an issue with the booster, I think that unless there's a leak, they tend to function well, practically forever. One thing you haven't checked (at least you haven't mentioned it) is whether the seal at the booster end of the vacuum line is intact and doing its job. If it can be easily rotated in the booster, chances are it's leaking air, and you should consider means to seal it.

Check your vacuum booster for master cylinder leakage (it happens). Remove the vacuum line and use a foot of clear plastic aquarium hose to suck out any brake fluid that may have escaped the m/c. Brake fluid may deteriorate the diaphragm in the booster, and conceivably cause a rust-up in there. This is of course a good time to check the vacuum lline seal.

Third. Your description sounds a lot like simply brake fade, where the rotor and pads get overheated because they can't dissipate braking heat well or quickly enough. Most of us have very little trouble with coupe brakes on stock or nearly stock tires. Physics. Braking transforms a tremendous amount of kinetic energy to heat. Think of acceleration which transforms chemical energy to kinetic energy (wasting a lot of energy as heat out the tailpipe and radiator). But your brakes transform all that kinetic energy to heat, much faster.

There are several ways to approach brake fade issues. You haven't said anything about the condition of your front rotors. If they're thin, there may not be enough metal to hold, or to dissipate, the heat of braking. This is suggested by your noting that initial stops are okay.

You also said nothing about the brake pads you're using. I expect you changed the pads when you changed rotors, but with what? Some pads are better than others, all of them trade off performance in one area or another. There are lots of places that will sell you pads and/or rotors. I like Balo rotors and Axxis pads, but that's me and not you.

You post that the one-way valve is "clear", I hope you mean that it flows freely in one direction only.

I think CNPR has rebuilt boosters or could direct you to someone who has them. Wouldn't hurt to give them a call, but I sincerely doubt your booster is at fault.

Concluding, check the seal at the booster, next rotors, then pads. Some exercise of the search function in these areas may prove useful.
 
Thanks, guys.

I'd never thought about fading as the cause, so that's worth consideration. I've had the car since '85 and used it as a daily commute car for a dozen years or so. In about '92, there was heat-related warping of the front disks, which I changed out for disks that are said to be more heat resistant. They were, and the warping stopped. The pads I've been using are the longer-lasting (harder) type, as opposed to the softer, faster-braking type of pad.

The hard-braking issue started in the mid-90s, when I was driving home on the freeway in stop-and-go traffic. (I don't think low-vacuum at idle speed is the problem, but I will watch for that.)

I haven't driven the car all that much over the past 10 years, but want to get it back into excellent condition, especially now that the weather has warmed up.

I just checked the vacuum-line connection at the booster and it felt solid. There hasn't been any noticeable drawing-down in the hydraulic fluid reservoir, so I doubt there would be leakage into the booster.

I will check the rotors and pads, and report back.

Thanks again!

Steve
 
Well I haven't checked the front pads and rotors yet, but the hard-breaking problem returned and so I just performed the following vacuum test (sent to me by a member of this forum):

"With the engine stopped, depress the brake pedal about 10 times. Keep the brake pedal held down and start the engine. If the brake pedal moves further down, the brake systems is in good order. If the brake pedal does not move, either the non-return valve, the vacuum hose or the rubber ring between master cylinder and brake unit may be defective or else the filter element is completely blocked."

The pedal did move further down when the car started, so I guess the brake vacuum/booster system is in good working order.

This makes brake fade caused by excess heat more likely.
 
Ok. I just checked the front rotors/pads and they look fine. The rotors are vented and show very little wear. The pads have plenty of material left.

I re-checked the connector at the booster end of the vacuum, and yes, it CAN be easily rotated in the booster, but so can the same connector on the booster of my '70 2800 CS, and the brakes on the 2800 work w/o problem.

If anyone has further ideas on this issue, they would be much appreciated.

Thanks.
 
I'd bleed the brakes first if you haven't done it in ten years or more. Just enough to get clean fluid out.

Then starting with the front- pull the pads and see if the pistons are stuck. ( Move the pistons slightly manually with a big screwdriver/ block of wood.) Clean all the dust, rust, crap that the pads slide on , new pins, etc. so you know nothing is preventing the pads from free movement.

Spin the wheels with it up in the air to see if one is dragging.

Check those wheel bearings for slop.

The old way to check for brake fluid op- open the cap and press the brake, you need a buddy, if there's a geyser, you need to bleed. A small jump of fluid is what you're after.

Rubber brake hoses collapse internally- you won't ever know till you check the wheel or it locks up with resultant heat.( glows like a racecar) Smell will help.
 
@ 61porsche:

This is good advice, tho a lot of work ... haha. Couple of questions & observations:

If the brake fluid has gone bad, wouldn't the symptom be soft braking, not hard braking?

The front rotors were replaced with rebuilds (from a 528, on the advice of a trusted mechanic) not too many miles ago because the pistons of the original ones were sticking. This caused pulling to the right (or left ... I forget) when braking.

Now, the car stays on a straight line when braking, so I would doubt that anything is preventing the pads and pistons from free movement (since there is no pulling to right or left).

Re: wheel bearings, there's none of the noise that's typical w/ worn bearings, and again, no pulling rt. or left on braking.

Of course, the front rubber brake hoses were replaced with new ones when the front rotors were replaced.
 
If it were mine...

I'd probably disconnect the line to the booster to see if the braking feels the same as when the problem occurs. Easy, and might help determine a direction to focus your efforts.

Second on my list would be hydraulic related components, starting with the master cylinder. They can do some really strange things!

Dan
 
It sounds to me that either your pads on the front can't take the heat of stop start driving, or your brake booster has a heat related fault, replace the pads first as that's easy/ cheap, otherwise the booster needs to come apart for an overhaul in my opinion
 
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