12 replies to this topic

[DClarke1954]

I have had to do some very sharp braking a couple of times.

Each time the same thing happens; the rear end drifts right which makes the front start to go left and forwards.

Surely this is not "normal"?

 

Any ideas as to why this might be?

 

[Quinlan minor]

Mismatched rear slave cylinders?

DAHIKT.

[nicklouse]

Sounds like one front caliper is being a bit lazy.

[mbolt998]

Mismatched tyres on the front can cause that. If one locks up before the other you can spin. But I'm sure there are many other possible causes. The front is probably the place to look rather than the back, because it's much easier to get a torque applied to the whole car from the front. If the rear is going to the right maybe the front right brake isn't coming on properly?

[bpirie1000]

What does the MOT rollers say about this or is it another exempt mini on the streets which may not be as safe as others.

[Lplus]

I have had to do some very sharp braking a couple of times.

Each time the same thing happens; the rear end drifts right which makes the front start to go left and forwards.

Surely this is not "normal"?

 

Any ideas as to why this might be?

I'd say the left rear brake isn't working properly.  Front brake problems lead to the steering pulling, but if the whole car slews one way without pulling the steering it's usually a rear brake.

[alpder]

Be worth identifying what brakes are fitted? Front/rear split, diagonal split, or single-circuit? And what type (if any) regulator for the rears? Servo, and if so on both or just fronts?

 

Certainly the first thing to do is get it checked on brake rollers. And one of the less-obvious things to watch out for is that the rears shouldn't create enough effort to be able to lock up - which would indicate a missing or mis-adjusted rear regulator. Once the rears are locked, any slight imbalance at the front would then cause the car to start to turn.

 

But it may not even be brakes: could be suspension which goes out of geometry when heavily stressed by braking hard. I had a car (not Mini) whose live rear axle (which is, of course "solid") had a loose joint at the diff casing and would steer the rear to the right on heavy braking. Took me a l-o-n-g time - and many fruitless brake replacements and test sessions - to find that one. Such an issue could occur on a Mini: sloppy rear wheel bearing or rear swinging arm bearing (or a slotted sub-frame without the washer-weld fix) on one side would potentially create similar conditions. Doesn't even have to be the rear: could be something like a tired/missing front tie-rod bush, but faults tend to be more obvious at the front so you'd probably have spotted it already, so (if brakes do get ruled-out) I'd start with jacking up the rear end and giving everything a good heave with a pry-bar.

Edited by alpder, 23 April 2025 - 10:08 AM.

[mab01uk]

What does the MOT rollers say about this or is it another exempt mini on the streets which may not be as safe as others.

 

I agree, easiest and quickest way to check this out is to take it to a local garage who do MOT's and ask them to put it on the MOT rollers used for brake testing, which will indicate which wheel/brake if any is causing the problem, or if the problem lies elsewhere.

Edited by mab01uk, 23 April 2025 - 08:59 AM.

[Ethel]

There are many possible causes. Presumably the brakes were reasonably balanced (cold) to pass your MoT.

 

The brakes themselves are still a good place to start. Give them a service to check disc & pad wear isn't too bad & comparable side to side, all caliper pistons move freely. The same with the rear drums, the pistons are unlikely to seize, but more likely to leak. Get a glamorous assistant to gently press the pedal so you can check the rears start to operate at about the same point.

 

If it's not the brakes, it could be almost anything suspension related. Tyres (pressure, condition & quality/make), dampers, rubber cones (springs), worn joints or bushes (including steering & subframes), bent bits chucking the geometry out.

[Lplus]

 

I have had to do some very sharp braking a couple of times.

Each time the same thing happens; the rear end drifts right which makes the front start to go left and forwards.

Surely this is not "normal"?

 

Any ideas as to why this might be?

I'd say the left rear brake isn't working properly.  Front brake problems lead to the steering pulling, but if the whole car slews one way without pulling the steering it's usually a rear brake.

 

Darn it - make that the right rear brake...

 

If one brake isn't up to scratch at the rear the working brake tends to pull itself into the centre.

[absx2]

If it`s not a leaky cylinder it`s most probably too much braking power going to the back wheels as this seems to happen far too often due the mismatches in slave cylinder bore configurations and dodgy brake shut off valves ( Fam7821 ) which is why I fit an adjustable rear brake bias valve like a MS72.

 

A leaking rear slave cylinder will make the brake shoe bite harder so that`s the first thing to check.

 

Probably some pictures or do you know if you are on a single line system with and limiter fitted to the rear subframe or a dual line with a shuttle valve on the bulkhead ?

[DClarke1954]

Thanks everyone.

It's not a historic car dodging MOT's, tax & ULEZ.  It dodges tax & ULEZ for sure......but has an MOT and was tested on brake rollers and passed very recently.

I need to dig into this issue a bit further to see if its a big mismatch across front pair/rear pair.

The car does have all new brake pipework etc. and is servo-assisted.

I will ask for a retest of the brakes across each axle (front/rear).

I just find it strange that it "jinks" arse-end out and right.  :)

[absx2]

Thanks everyone.

It's not a historic car dodging MOT's, tax & ULEZ.  It dodges tax & ULEZ for sure......but has an MOT and was tested on brake rollers and passed very recently.

I need to dig into this issue a bit further to see if its a big mismatch across front pair/rear pair.

The car does have all new brake pipework etc. and is servo-assisted.

I will ask for a retest of the brakes across each axle (front/rear).

I just find it strange that it "jinks" arse-end out and right.  :)

 

It`s no surprise it passed an MOT recently as unless they are very familiar with classic minis a good strong reading on the rollers is a pass.

More likely it locked up on the rollers which is also a pass but in reality it`s a dangerous scenario that needs addressing on a ten foot long car with most of it`s weight on the front.

Raise the rear of the car and have an assistant press the brake pedal then give the rear wheels a good hard turn and you should be able to turn them with a good level of resistance.

Try different levels of pedal pressure but if the rear wheels are locked solid off the foot brake there is your problem. It could be wet/leaking or the wrong cylinders or a dodgy limiter valve.