bmwe21.net forums

The brakes are seizing up on my 323i M50 whilst driving along... When the car is cold they are fine but after 10mins of driving all four brakes are locking up. If I leave the car to cool down they unseize, I find it hard not to think it's not heat related. It's definitely all four since when they locked up tonight I jacked all four corners up straight away. I'm running standard E21 pedalbox/linkage/servo/master cylinder, Wilwoods up front with brand new fast road pads, and E21 rear calipers with standard pads with plenty of life left in them, discs are also all fine.

Thoughts so far have been
1) Faulty master cylinder
2) Faulty linkage holding the brakes on
3) The brake pipes are too close to the head (a few MM or even touching in places) and are getting too hot, locking the brakes up
4) Servo might be too close to the head, but would this really matter?
5) Brake pipes are getting to hot somewhere else on the car eg exhaust manifold, but I don't think this is the case.

I checked out 1) tonight, master cylinder feels fine and feels better than other known good MCs I have, but perhaps the MC has other issues?

I thought 3) might be a possible cause, but would the head give the brake pipes so much heat that all 4 corners are locking up? Also the clutch is fine and that's routed right next to the head as well. The front drivers side brake is also locking up, and that goes no where near the head.

I'm tempted to route the pipes inside and perhaps even fitting an aftermarket pedal box much sooner than anticipated...

Any ideas??

For all four to be effected I reckon it's gotta be master cylinder related. Is it the first time you use them that they jam on? Have you checked the valve in the servo hose too?

did you fit the vacuum pipe valve that gos from the rocker cover to the brake booster the right way round? i think its building up pressure and then sticing - only plausible explanation i can think of!

Interesting theory guys... maybe not. The servo hose was dumped on without much consideration!

Lol does that mean you don't even have a one way valve!?

It sounds like the only feasible reason I could come up with and explains why after a few minutes running the brakes come on! It's the only connection between brakes & engine?

nas80 wrote:Lol does that mean you don't even have a one way valve!?

Nail. Head.
As a back-up theory, if it is heat-related, let the car get up to temperature and check where the brake lines are near to a heat source.
A faffing big M50 engine has to be the main culprit.

I think the servo pipe is #1 on my check list then...

The pipes are extremely close or even touching the back of the M50 head, but I've been told that engine heat is unlikely to effect the fluid. Plus the drivers side brake is also locked solid which is not routed near the head (the pipes to the MC were not that hot), the clutch is routed next to the head and is behaving as normal.

I didn't have this problem my lines run in the stock location as well.

M50 isn't a hot running engine. I struggle to get up to temp sometimes.

My gauge is reading halfway/over halfway using a brown temp sender from an M42. Where's your's sitting and how do you know yours is running cool?

Mine sits at a quarter usually and only goes halfway when sat in traffic. As soon as you move off it drops back down to a quarter.

I'm using the e30 brown sensor from an m20. It's the correct one for my gauge.

I know its running too cool, cos there's a lot less heat from the heater when your moving esp on the motorway. I know its cos of the ridiculously over spec'd rad. I stuck a bit of card board behind the grill it helps a lot.

After a long run in traffic I've opened the bonnet there's no heat blast like I used to get in the rx7 its just a bit of warm air.

At least I know I'll be fine on the one week of the year where we get hot days!

Someone else had this problem a couple of years ago. Search the forum - I'll see if I can find it myself but am cooking Wife's birthday dinner.

Bingo!

Seems he fixed it by replacing M/Cyl and rear brakes.

http://www.bmwe21.net/forum/viewtopic.p ... ing#p21871

Servo and M/C would be the only things for Rob - he's refurbed the rest. Of all the people on the forum Rob must have spares to try. Plus the vacuum valve thing to try.

I reckon it's the M/C myself - sticking seals.

Cheers guys. I'm not sure if that guy said his front brakes were locking as well? Plus he had drums too.

I personally think the MC is fine as far as I can tell, but I'll swap it off for another one anyway. Since I've got the irritating servo bar off I might as well swap the servo as well, though for a replacement it might be harder for me to know which is a good servo, which isn't and which ones haven't previously been sat outside in the rain . The linkage will be greased up and checked and I'll check that servo pipe too.

Looks like I'll have some brake related fun tomorrow evening!