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MG MGB Technical - Quick brake question

I'm relaly after confirmation that I'm on the right track with my problem diagnosis.

1970 B GT, no servo.
Recent fliud (dot 5.1)
Recent recon callipers, braided hoses.
New meal brake pipes throughout the car
Recent rear brake cylinders.
No fluid leaks.
Plenty of meat on the front pads and rear shoes.

The pedal is long and pretty mushy. Get past this and the car does stop but there is no inital bite to the pedal, and ultimate stoppping power is much reduced over normal.

The fluid has not been boiled and the brakes have not been over heated as I simply can't use them hard enough to get enough temprature into them for this to happen. Master cyliner is topped up and the level does not go down as the car is used.

I recently bled the system and got a lot of air out (this was after changing calliper and hoses), but the pedal was still not brilliant.

So I intend to re-bleed again, probably ending up with a full fluid change. Starting with the rear near-side brake (UK car) and working to the front off-side. If the pedal is still mushy am I right in thinking that the problem is probably the seals in the master cylinder?

I've had the car for many years, and the brakes alwasy felt very good - a bit like trying to squash a brick with your foot. I'm trying to get back to this level.

If I sit in the car and just push the pedal you can push it a considerable distance down - I sem to remember that when the brakes felt good this was not possible.

Pumping the pedal makes little difference to the feel of the pedal, perhaps adding a small amount of firmness but that is all. I've checked the pedal box and there is no flex or movement here when the pedal is pressed.

Thanks in advance for any info....

Tony
T Crossley

I've always had a problem bleeding both my roadster and V8, and the more radical the work in the braking system the worst it is. At the very least I have used a gunsons EeziBleed to fill the system and bleed the rears OK, but the fronts have always needed an extra step whereby someone stands on the pedal while I rapidlky open and shut the caliper bleed nipple. This has always got an extra 'lump' of air out each side. When I removed and refitted the servo I had to do this a couple of times. Without a servo the pedal should be harder, and there should be no tendency for the braking point to 'pump up' with repeated applications of the pedal. New pads on old discs, or possibly new shoes in old drums, can cause a sponginess without pumping up simply because the two surfaces need to bed in over time.
Paul Hunt

While we are on this subject I would like to ask one small question. Our 77 B has been parked since 97 and I have forgotten what a normal brake peddle should feel like in this car. To me the peddle feels soft and the braking power less than adequate. I can lock up both the front and rears but with extreme effort as compared to my 95 Toyota and 99 Suburban. My question I guess is; Should I expect the brakes on the MGB to feel the same as my other cars?


Thanks,
Kevin
Kevin Pearce

Tony,

Sounds like a simple case of rear brakes not in proper adjustment. And as Paul says, the arc of the new shoes may not match the arc of the drums, so you may not be able to adjust them properly until they've worn to match.

Time ago you could have your shoes shaved to match your drum. I doubt any machine shop does this anymore. But if it annoys you enough, you may want to phone around and see if you can find one who still does.
Matt Kulka

One thing to check on the rears is that the adjustor holds OK. I've experienced one with stripped threads which meant it seemed to set alright but got pushed in whenever you applied the brakes hard, giving the symptoms you describe.
Steve Postins

Tony,
Again check the rear brake adjustment but more important you are using synthetic brake fluid DOT 5. That's an aeroscopic compound. In short, it holds air and it is extremely difficult to get all of the air out of the system. Before I went ripping and tearing at the master I would exercise a bit of patients and look for those tiny air bubbles in the master. My bet is you are still full of air.
Roy
Roy Dougherty

How old are your hoses?
R. L Carleen

I removed the brake booster, [power brakes] from my '77 to accomodate the larger K&N air filters. With the earlier brake/clutch master cylinder setup. To properly bleed the system, I've had to
'bleed backwards'

Explanation; open each bleeder screw individually, and using a hand oil pump, force brake fluid back towards the master cylinder. This is the only way that I have found that works 100% with clutch cylinders as well.
This of course is preparing the pump by cleaning anything but brake fluid from the pump. And then using, preferably a clear hose from the tip of the 'oiler' to the bleeder screw.

This gave me a very good pedal.
Safety Fast
Dwight McCullough
Dwight McCullough

Kevin -

In answer to your questions, I would not expect the brakes (even the servoed ones of the later cars) to feel like a modern car. Modern vehicles (especially Japanese in my experience) are rather over-servoed and light.

However, the pedal shouldn't feel especially soft: it's possible that your brake fluid has taken on water while the car's been standing (normal brake fluid being hydroscopic): this will cause it to compress under pressure and could explain the spongy feel. I'd recommend changing the fluid, and having a good look at the rubbers, especially on the rear cylinders.

Hope this helps
Bruce
bruce

Thanks fella's. The rear shoes/drums haven't been replaced recently (about the only thing that hasn't).
(Shoes drums are in good condition after checking)

Interesting advice about the rear adjusters ,I've often had problems adjusting the rear brakes and getting it all satisfactory even before the brakes took their recent turn for the worst.

I'll also give Paul's bleeding method a try and see if it improves.

Interestingly I've just swapped the front pads and the compound is braking up in many places and generally looks very poor despite never having never been 'cooked'. Almost like the compound was poorly bounded to the backing plate. Some sections I can easily crumble off just by rubbing the pads together.

I'll bleed it all again, bed in the new harder compound pads I've now fitted and see how it feels. I've got a little under 3 weeks to get this sorted. Then I'll be racng again at Croft circuit - and as this includes several laps of having to scrub off almost 120mph you can understand my desire to get this pedal feel and bite sorted!

Tony

Tony Crossley

MGB brake w/o servo should be like stepping on a brick, as you say. Bad rear adjustment gives a lot of pedal travel, but no squishiness once the clearance is taken up. A binding caliper (or loose wheel bearings) will give the effect stated. Remove the wheels, and check to see if there is a noticeable clearance on one side of a disc. Watch very carefully as someone stands hard on the brakes. If you see deflection of the disc (or caliper), there is a stuck caliper piston. When released, the wheel should turn freely, with very minor drag, and there should be no visible clearance twixt pad and disc. Are the pads disintegrating equally, or just some of them? If all the same, there is something wrong with the material, if different - sticking piston/pad. The next step after bad brakes with stuck piston is a fractured disc.
I would ditch the silicone fluid and use GTLMA. Despite claims of suitability, the stuff is compressible and more so at high temps. Tilton Engineering cites reports of corrosion under some conditions as well. "NO major team that taxes its brakes uses silicone because of its compressibility" Silicone's expansion can cause a hydraulic lock which holds the brakes ON, with overheating which would explain both the bad brakes and the pad disintegration. Be sure you have clearance on the MC pushrod, as no clearance will do the same.
FRM
http://www.usachoice.net/gofanu
FR Millmore

Hello again, I noticed when swapping the pads that the drivers side disc could be moved fractionally - IE, the bearing is not fully tight (or is less tight than the passenger side).
I can tug on the disc and feal it move slightly - even if I can;t see it. So I suspect this 'aint helping!
Having said that, once I applied the brakes for the first time with the new pads I can no longer move the disc in the same way. Turning the disc does give just minor drag on the new pads.

Just to clarify a point a few people have made, I'm using dot 5.1, not dot 5. Dot 5 is the silicon based stuff, 5.1 is actually a higher temp version of dot 3 or 4.

All 4 pads were falling apart so I',m pretty sure they were not well manufactured.

How do I check clearence on the master cylinder pushrod?
T Crossley

I see you've replaced the brake lines with braided hoses. Did you do the rear hose too?

You say the pedal does not "pump up?" That should eliminate mis-adjusted rears as a cause.

I've found that a mushy pedal is often the MC.

Having a mis-adjusted front bearing clearance or two can't be good. The new pads keep it in place but re-packing and adjusting would be wise, considering where you will be playing soon.

Mike!
mike!

Tony, The disc should move slightly, if the bearings are adjusted correctly, and both sides should be the same. Adjust bearings as per the shop manual. It must be done with NO grease in the bearing, NO seal. In the shimming process, tighten the nut as specified, even to the point of putting the split pin in. The spacer will compress enough to throw off your readings, if you don't do this. Correct end float is .002-.004, which is JUST feelable, but usually NOT visible. The movement at the disc OD will be more and MAY be visible. AFTER you get the shim thickness figured, take it apart and grease the bearings and fit seal. You will NOT be able to feel the clearance with new grease in the bearings. You have no business going this fast with bearings loose OR tight!
What brand of fluid is it, and what are the wet and dry boiling points? I am not familiar with the DOT 5.1, but AP Racing fluid is 550F dry (possibly more now, that's an old spec), wet not specified as you should be changing fluid frequently in this application.
On MGB, the pushrod clearance is built into the cylinder, if the pedal is not binding up or somesuch. Check that you can wriggle the pedal without pushing the rod in. You do not mention a master cyl rebuild/replace. I agree with Mike! that this is a common cause of this trouble. The MC can suck air in, ruining all bleeding efforts; or, swollen seals can cause the same hydraulic lock as no pushrod clearance.
For everyone's benefit, take the pads back to the MANUFACTURER. They need to know if there is a problem, they may be able to tell you why they failed, and someone else's life (maybe mine!) may depend on this knowledge. At worst, you may get your money back. Again, what is the brand/compound?
Check to be sure that "rebuilt calipers" have the correct bolts holding them together. Weak bolts here can cause caliper flex and could be fatal.
For some demented reason, the rear wheel cylinders on MGB are mounted upside down, with the bleeders at the bottom, which makes it impossible to get ALL the air out. It only works because the pistons contact each other, leaving very little space for air - IF the brakes are adjusted correctly. If the handbrake cable is holding the brakes ON a bit, the pistons do not contact each other, leaving room for enough air to screw up the brakes, and you cannot get it out short of turning the car over. DISCONNECT the handbrake cable, adjust the brakes, bleed the system, then reconnect the cable when all is well. I frequently lock the rear brakes with the adjusters when bleeding, to be positive the pistons are in contact. With the adjusters locked, if ALL the air is out, you will have maybe 1/2 inch pedal travel and ROCK hard!
Bleeding by pumping mixes air into the fluid, which may take hours to come out. Use of a vacuum pump sucks air past seals (they are designed to seal pressure from the INSIDE, not outside)- you can bleed forever and keep getting air, but no brakes. High pressure on the MC causes cavitation and mixes air in. Low pressure on the MC reservoir is fine. I now bleed most cars by gravity, AKA very low pressure. Once you have some fluid in the system, just open a bleed screw and let it run by itself. After bubbles stop showing, go to another bleeder. KEEP the reservoir FULL! Not all cars can be done this way, but it works fine on MGB, and you can do it alone. If you have to pump to get the gravity bleed started, let the thing sit for several hours, then gravity bleed again.
Good luck, let us know what happens!
FRM
http://www.usachoice.net/gofanu
FR Millmore

Phew, after several hours last night I now have a much better pedal but what a catlogue of problems.

Firstly bleeding, I got a massive amount of air out of the system.

Secondly, I noticed the pedal box was moving slightyl as the pedal was pressed harder. Reason -two missing bolts holding the pedal box to the bulkhead! Fixed those and noticed the box still flexed. Reason - the stifferner running above the the master cylinder pistons was missing! So sorted that.
I also spotted that the MC piston was coming in at a slight angle due to the spacers on the pedal pivot not being correct - so sorted that as well.
Then spent time with the bearing and that is now correct.

So those were sorted but still the pedal felt odd.
Checked the calipers to see if the disc was distorting, etc as mentioned above. It wasn't, but the damn caliper was!
The top cross bolt wasn't tight and the calliper was flexing apart about 2mm. Certainly enough to get the flat blade of screwdriver between. Mother of mercy!
So, tightened that up to 3 grunts torque and unsuprisingly the pedal got a whole lot harder. (Both front callipers will be less than a year old and have convered maybe only 500miles. These were supplied by Moss who have an outlet locally. I shall be having a few words!)
A few more tweeks here and there and now the pedal feels much more like it's old self.

I just need to stick the carb back on now(another story) so I can give it a road test.

Thanks for the information and advice so far guys, it's been much appreciated.

Tony


T Crossley

Tony,
Insist on ALL NEW correct caliper bolts, and find out the correct torque. After driving with some loose, they are somewhere on the road to fatigue failure from bending/overload. Maybe 10%, maybe 99%! You don't want to be thinking about this as you go off road at speed. Sounds to me like you had best do a thorough going over of all critical bits on this car. Sloppy work rarely only affects one subsystem.
FRM
http://www.usachoice.net/gofanu
FR Millmore

I'm with FR on this one. Boy, my hair is still standing on end!

You cannot be too safe with brakes, especially with the speeds you are contemplating.

I am SO glad you found those items and got them cleared up... everybody listening???

Find those bolts and have fun at the races!

Mike!
mike!

This thread was discussed between 03/09/2004 and 08/09/2004

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