Brake Problem

[zuffen]

In the Shed I have a Thread on the MightyBoy build.

I took it for a drive this morning and the brakes are woeful!

The pedal is firm and the rear brakes work well. In fact they are the ones doing all the work.

The fronts were warm but cool enough to touch after a 6k drive.

The car has Nissan N14 Pulsar discs each end.

The front discs are new DBA's and the rear are what was on the donor.

All callipers have been rebuilt, all new pads, it has braided brake hoses and runs the N14 pressure distribution valve, that is only plumbed into the rear lines OEM, so shouldn't effect the front.

The Master Cylinder is for a Nisan Pulsar GTi-R as they are non ABS but identical dimensions . Apparently there is a difference between ABS and Non-ABS master cylinders. Who'd have guessed that.

So when trying to stop the pedal is nice and firm but there's no risk of locking up any wheels.

I'm open to all suggestions.

[46 deluxe]

Is the booster ok ?
Does the pedal drop a fraction when you start the car with your foot on the brake pedal ?

[zuffen]

You're onto something there.

I was fiddling today and found the booster wasn't getting any vacuum.

I turned the one way valve around the right way and the front brakes now get hot from use. I need to assume I put the valve in back to front so my bad.

That's progress.

The braking still isn't stellar and I'm going to do some testing on the booster as pedal pressure still seems high.

I guess if I keep searching I'll find the culprit.

[Gojeep]

Is the pedal ratio also the same as Nissan Pulsar GTi-R?

[Greg N Smith]

I can find Master Cylinder Bores of 20.64mm and 22.22mm for Pulsar

22.22 pushes 16% more fluid than 20.64

Are your Calipers and Master Cylinder compatible ?

Have you changed the Front/Rear weight distribution ?

Happy Hunting !

[zuffen]

Marcus, Pedal ratio the same.

Greg, The GTi-R master is a direct swap with the N14 master so all the hydraulic bits match each other.

Now front/rear weight is way different to factory.

The Pulsar weighs around 1080 and the MightyBoy maybe 700/750. Most of the weight is on the front axle, I'd say 70% or more.

I'm thinking the booster may have an air leak that is not giving full vacuum. The idle has a lope like it has big cams in it. It could well have cams as we purchased a sort of non-runner as a donor and have no idea what the engine has in it. The car came with 3 spare engines so we weren't too worried if the engine in the car was a dud.

The thing goes really well, as in silly fast for one of these, but it would be nice to get it stopping better.

[Harv]

You may well be on the right track with the vacuum being depleted by valve overlap. How much vacuum does a gauge read? If its down around 10-12mmHg then the booster may not have enough suck to do the job.

In the spare parts department, do you have an electric brake vacuum pump? Temporarily plumb it in and see if 18-24mmHg gives it better stoppers. A Mightyvac type hand pump might do the same, but you will only get one brake application to check before having to pump it up again (would have to drive it on the handbrake in between).

A softer organic pad (non-metallic) may help a little, though will wear fast (and the dust will drive you nuts).

What does the internal diameter of the braided hose look like? Good hose has a fairly large internal bore, but some hoses have a tiny teflon liner despite having bulky stainless braid. The small bore can act almost like a proportioning valve.

Cheers,
Harv

[zuffen]

Harv,

Interesting observations.

I'll put a vacuum gauge on it and see what it pulls. That I do have in inventory.

The vacuum pump, I have one but it's on another car and will be staying on it.

I'll certainly be chasing the vacuum angle as I think that's the issue. I'm starting to wonder if it has a leak around the master/booster interface. That would reduce the vacuum available and could lead to the odd idle.

The brake hoses are new, they've now done 10kilometers! I haven't looked inside them and hope I don't have to.

The pads I'll wait until I sort vacuum out and see what happens.

The driveway the car lives in is road base and it can lock the wheels on that but not on hotmix.

No test drives the next 2 days as they are resealing our street so easier to stay off the road.

[46 deluxe]

It should take 3-4 pumps of the brake pedal after engine is switched off to deplete the vacuum and return to a hard pedal .
If the pedal is hard straight away after shutting the eng down , the booster is rooted .

[zuffen]

Didn't do much on the little car today.

I replaced the vacuum line from the manifold to the booster as I couldn't be sure of the makeup of the hose used. If it wasn't designed for vacuum it may have been collapsing.

I may try backing off the master cylinder form the booster and seal between them to ensure there are no air leaks.

I have plenty of other things going on so this may take a few days to sort.

I appreciate the help and every little bit gets me closer to the end goal.

[Harv]

Another thought, and perhaps a long shot: what booster did you mate up to the Nissan Pulsar GTi-R master?

If it is not the GTi-R master, then there may be a difference in internal pushrod length (not the pushrod that hangs off the pedal clevis). If the pushrod is too short or long it can change the timing that the internal ports are covered/uncovered.

This is one of the dramas when mating up the XA/B/C Ford master to a Gemini booster - pushrod looks OK, but needs to be lengthened to 74.5mm to preload the assembly.

Cheers,
Harv

[zuffen]

Harv,

I've been pondering that but don't want to think about it

The booster is the Daewoo Matiz unit and the pushrod length measured up OK compared tot he Nissan item.

I think the timing of the ports operating is OK as it has brakes just poor brakes.

I haven't touched it today and may not get time tomorrow.

Whilst the car is driveable it isn't engineered so I'm a bit cautious where I take it. I wouldn't fancy having an accident in it.

The reason I don't want to think about the pushrod length is it's a mission to get the master cylinder off and out of the car and to remove the booster is a dash out exercise I've done too many times to want to do it again.

[FRANK BASILE]

What Harv describes with the XA-XB master cyl is a problem created because those master cyls have no piston return limiting circlip . They require around 1mm of preload from the booster pin. Too long a pin will block the compensating ports,too short and you risk fluid leakage back from the piston. In this current set up you will have a circlip and the booster pin should be long enough to just neat seat and not engage the piston. This will give if it is too long a harder pedal and sudden engagement of brakes to the touch. Too short and you end up with excessive pedal travel before the brakes engage Hope this helps.

[zuffen]

Thanks Frank.

Been too busy with other more important work around the place.

Hopefully will get back on it in the next week, or maybe over Easter.

[jeffa]

Another thought, and perhaps a long shot: what booster did you mate up to the Nissan Pulsar GTi-R master?

If it is not the GTi-R master, then there may be a difference in internal pushrod length (not the pushrod that hangs off the pedal clevis). If the pushrod is too short or long it can change the timing that the internal ports are covered/uncovered.

This is one of the dramas when mating up the XA/B/C Ford master to a Gemini booster - pushrod looks OK, but needs to be lengthened to 74.5mm to preload the assembly.

Cheers,
Harv

Off topic, but in case someone is searching and finds this: A Toyota Crown (UZS131) booster uses an adjutsable pushrod. It fits the Gemini booster and can be adjusted within the length required to fit the Gem booster onto the XB M/C.