Rear brakes over-heating, possibly fading on 3.0CS

[taylorcom]

I took my '72 3.0CS for a spin today (a 6.5-mile roundtrip to get gas) and when I got within about half a mile from home on the return trip, I noticed the brakes fading. I got home, checked the fluid reservoir and it was full. Then I checked the rear wheels and both were very hot. So I'm thinking the brake fade could have been due to over-heated hydraulic fluid at the rear brakes.

The right rear caliper was replaced with a new one in the mid-90s, the left rear caliper and hose may be original. I've noticed that new calipers are hard to find now, so would it be wise to start by replacing the hoses and see if that improves the situation? Thanks in advance, folks.

 

[Thomas76]

If both rears were hot it doesn't seem likely that they both failed at the same time. I would consider things that provide fluid to the rear set first.
1. Hand brake dragging a little?
2. That device that sits near the differential? Someone will know the name of it here.. (flow divider maybe?)
3. Master cylinder?

 

[taylorcom]

If both rears were hot it doesn't seem likely that they both failed at the same time. I would consider things that provide fluid to the rear set first.
1. Hand brake dragging a little?
2. That device that sits near the differential? Someone will know the name of it here.. (flow divider maybe?)
3. Master cylinder?

The left rear has been heating up for a while now. The right rear heating up, and the brake fade, are new developments.

I checked the hand brake lever, and don't think the brake is dragging.

I didn't know about the device near the diff. It seems worth a look.

Master cylinder? Good question. How would I check that?

 

[Dick Steinkamp]

An old brake hose will swell internally to the point that you can force fluid through it when you apply the brakes, but the fluid will return very slowly or not at all keeping the pads against the rotors.

An original brake hose on a 50 year old car is not a good thing. If your hoses are over 6 years old or you don't know how old brake hoses are...replace them. They are inexpensive and it is a relatively easy DIY. In the process you change the brake fluid which should be changed every couple of years anyway.

If your car doesn't go...it's inconvenient. If it doesn't stop....it's deadly.

 

[Dick Steinkamp]

The thingy near the diff is a brake pressure regulator. It limits the PSI going to the rear brakes to help prevent the car changing ends during a panic stop.

 

[Thomas76]

Not sure how to verify master cylinder exactly. It is one of the most expensive components to replace however.
If you've been battling one side for a while then Dick's advice makes more sense as a starting point.
Maybe rusty parking brake components are fighting you?
Get the rear end off the ground, apply brake pedal and release. Turn the wheel, then crack the bleeder on the caliper and turn again. If it's a hydraulic problem it will spin easier.
Hope this makes sense.

 

[taylorcom]

Not sure how to verify master cylinder exactly. It is one of the most expensive components to replace however.
If you've been battling one side for a while then Dick's advice makes more sense as a starting point.
Maybe rusty parking brake components are fighting you?
Get the rear end off the ground, apply brake pedal and release. Turn the wheel, then crack the bleeder on the caliper and turn again. If it's a hydraulic problem it will spin easier.
Hope this makes sense.

Yes, makes sense. Good idea to check parking brake components, too.

 

[halboyles]

Time to replace all the flexible brake lines on the car. It's inexpensive and easy (mostly). Then you can focus on other components knowing that the lines aren't the problem. Here's what a 40-year old rubber brake hose can look like inside.

 

[Thomas76]

That's a great picture!! Thanks.

 

[bluecoupe30!]

Time to replace all the flexible brake lines on the car. It's inexpensive and easy (mostly). Then you can focus on other components knowing that the lines aren't the problem. Here's what a 40-year old rubber brake hose can look like inside.

Great photo! This is a common problem on all of our old cars. I had this happen to the flexible clutch hose in my Austin Healey. Really simple fix. And as Dick says, replacing hydraulic fluid regularly is recommended preventive maintenance.

 

[Cornishman]

Taylorcom,
Personally I would dismantle the rear brakes completely, check out the handbrake, calipers, shoes, pads, change all four wheels worth of brake the hoses and of course the fluid. Sounds to me like there are several issues going on. If any of the metal pipes look poor, or don’t want to undo, change those also.
Whilst at it, change the clutch flex hose and make sure that is also flushed as it shares the same fluid.
They are a pain to bleed, so do it all, once and properly. I did mine in two batches and wish I hit it all first time around.

Then, if still a problem start to look at the master cylinder and that pressure regulator.

I’m not sure how a left hand drive servo works, ie which wheels it does, so someone else will have to advise on at which stage to look at that, but I guess after the first focussed on rear end.

 

[pickman]

The valve on the differential acts as a proportioning valve it too must be functional. During my rebuild this valve required disassembly and refurbish as it had got pretty corroded. If your brakes are truly 40 years old the proper regime is to replace the master cylinder, all brake hoses, confirm all calipers are properly working and not leaking (best to rebuild them). From a functional vehicle standpoint brakes are a "SAFTEY" system if not in proper maintenance may cause personal injury to you or others. Brake fluid is hydroscopic and absorbs moisture it too need to be changed at regular intervals.

 

[taylorcom]

The valve on the differential acts as a proportioning valve it too must be functional. During my rebuild this valve required disassembly and refurbish as it had got pretty corroded. If your brakes are truly 40 years old the proper regime is to replace the master cylinder, all brake hoses, confirm all calipers are properly working and not leaking (best to rebuild them). From a functional vehicle standpoint brakes are a "SAFTEY" system if not in proper maintenance may cause personal injury to you or others. Brake fluid is hydroscopic and absorbs moisture it too need to be changed at regular intervals.

I do have a rear-brake caliper repair kit. Can you offer any tips on rebuilding them?

 

[Dick Steinkamp]

I do have a rear-brake caliper repair kit. Can you offer any tips on rebuilding them?

If you have never rebuilt a brake caliper, It will be difficult to explain the whole process easily here. Search on this site for "Caliper" for some tips. Best would be to get the Bmw Workshop Manuals (Blue Books). Chapter 34 gives a good description of the rebuild process. However, if you do not generally do the maintenance on your car, this is probably not the task I would start with.

 

[taylorcom]

If you have never rebuilt a brake caliper, It will be difficult to explain the whole process easily here. Search on this site for "Caliper" for some tips. Best would be to get the Bmw Workshop Manuals (Blue Books). Chapter 34 gives a good description of the rebuild process. However, if you do not generally do the maintenance on your car, this is probably not the task I would start with.

It's not a job I'd relish, though I had a Triumph TR-6 for many years on which I rebuilt the brake master cylinder (probably more than once). It worked okay, though not like a new one.

 

[bill]

To support what others have said, it's the hoses. Time to be replaced. I Had the same problem.

 

[Ohmess]

Taylor - if you are going to rebuild the brake calipers, don't leave the brake lines open at the rear of the car for any length of time. When the brake hoses deteriorate, they can allow small bits of rubber to get into the brake fluid. You can flush these out when you bleed the system, but if you work like most of us you will first take the caliper off, rebuild it, install the rebuilt caliper and then bleed. The problem with this is that as brake fuild weeps out of the rear of the brake system, any small bits of rubber in the line flow upstread into the brake proportioning valve. Once they get in there, the valve must be rebuilt or replaced.

Clamp off your old rubber hoses with vice grips while rebuilding your calipers to avoid this. Better yet, bleed the rears before you disassemble them, then clamp the old hoses, then replace the hoses when installing the rebuilt caliper.

 

[matthewp]

I had the same problem. I replaced the hoses, rebuilt the callipers to no avail. Turned out to be the pressure regulated. Perfect now.

 

[taylorcom]

Progress so far: I just replaced the right rear hose. The old one was so clogged I couldn't blow air through it. Left rear hose is next.

 

[m_thompson]

Progress so far: I just replaced the right rear hose. The old one was so clogged I couldn't blow air through it. Left rear hose is next.

I had the same problem with front hoses while we were in the middle of a BMWCCA tour. I replaced all of the hoses.