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MG MGB Technical - Bleeding Brakes,,How?

I have a 79MGB that I have owned for 5 years... with antique plates, drive about 1,500 miles a year UP TILL NOW!! (Total milage is about 72,000 miles) I am planning on some trips of about 500-600 mile range and I know the miles should go to about 5,000 a year.

I've notice the brakes fill a "little" spongy on the trip to Watkins Glen... not sure if it is because I am so use to power brakes on other cars I drive.

The last time I had it looked at (1,000 miles ago) the mechanic said I had "good" brakes, pads, and rear brakes.

So, I think I need to bleed the brakes and I have a few questions that I need help with:

1. Should I change/drain the current fluid? If so.. tips on how to do it. Never been done for the last 5 years.
2. Type of brake fluid to use?
3. Do I need to have the motor running at the time I bleed the brakes so the "booster" is working?
4. Where do I start to bleed the brakes? I had an old 46 Ply coupe in 61 (Yeah 1961) an I always started on the right rear, left rear, right front, left front. Will this work? Or what's better?
5. Should I change the flexible hoses too at this time?

Any help would be usefull.... is this something I can do with just "basic" tools?? Opps another question.

Looking forward to your replies,

Drive safety fast (with good brakes of course),
Donald Wheeler

Hi Don

I just finished this on my 67.

Brake fluid does not really go bad. It get contaminated or burnt. If it is clear it is probably ok. If it is brown or tea colored, it needs to be changed. It is just a bit more work if you want to drain the whole system. A couple hours verses a half hour if you are just bleeding the existing system.

You should not need to have the engine running. But since my GT does not have a booster, I will leave the final word on that to someone else.

As to the type of fluid. If your's is clear, it is either DOT 3 or DOT 4. There are a number of threads in the archives and I believe one on either the MGB technical or general page.

If your fluid is purple, it is DOT 5 (silicon).

I was always taught to start at the break farthest away from the master cylinter to asssist in getting all the air out. Not sure if that is just a mechanics story or if it has some fact base, but it is as easy as any.

If you are simpley trying to stiffen the pedal, a simple bleed is probably ok for now. If you are going over the whole break system, then changing the fluid, hoses and probably new cylinder kits are the best way to go.

This is a reasonably simple job. You can purchase bleeding kits that make it a one person job, but it is probably the easiest to do with two people.

Other than the wrenches for the bleeders, a hose and a bottle are all that you really need.

One word. If you have clear fluid (DOT 3 or 4) take care not to get it on any painted surfaces.
Bruce Cunha

Don; Your car should have a Pressure Differential Warning Actuator in the master cylinder. If you look in the Haynes manual at chapter 9 section 7, page 149, it tells you to unscrew the PDWA 3 1/2 turns prior to bleeding the brakes. Also for some reason they say to start at caliper nearest the master cylinder.

I don't know if the sequence matters, but I watched a friend spend a half day trying to bleed brakes on a Triumph TR 8 with a PDWA, he forgot to loosen the PDWA. It worked fine after he followed the instructions.

If changing the fluid you can speed up the process by sucking the old fluid from the reservoir with Mity Vac or syringe. Fill with fresh fluid and bleed. It saves a little time.

Clifton Gordon

Don. First, you need to determine what fluid you have in there. Most of us use Castrol LMA brake fluid. Others use Silicone as Bruce has mentioned. They are not compatible and the system must be flushed, throughly, before changing types. I use the LMA in all my cars.

As to your first question, yes, if you have non-Silicone brake fluid it should be changed every couple of years. It absorbs moisture which can cause rusting in the master cylinder, wheel cylinders, front calipers and, under some circumstances, the brake lines themselves. Drain the MC as per Clifton's instructions, refill with good fluid and bleed the brakes until you are getting good, clean brake fluid into your catchment jar.

Yes, changing the flex hoses is an excellent idea if you do not know they have been replaced within the last five years or so. Rubber will deteriorate over time and this is not a good thing to discover when driving, especially in traffic or during a panic stop. Changing them out is cheap insurance. There is also a copper washer involved and not all brake hoses come with it. Look at the Moss catalog illustration and order a few extras as a "just in case". Old copper washers can be annealed and reused if they are in good condition, but you do not know their condition until you are into the job. Hence, purchase new ones.

The MGB was designed as a right hand drive car and adapted to left hand drive. The furthest wheel from the master cylinder is the left, rear wheel. I have always bleed it first, followed by the right, rear wheel, right, front wheel, then left, front wheel.

No, you do not have to have the engine running to bleed the brakes even if there is a power booster.

Much information in the archives on this subject if you need more information and other methods of doing the job.

Les Bengtson

Just curious, why does the pressure failure switch (aka, Pressure Differential Warning Actuator) need to be backed off 31/2 turns? I've never done it and have had no problems (e.g., tripping the shuttle value). Could my PDWA switch be defective (i.e., no warning light)?

While you are at it,change out the rubber flex lines for the braided steel type. It will make a huge difference to how solid the pedal feels.


Steve, I used the Haynes manual as a reference, perhaps it doesn't matter. I have no direct experience with later cars and I'm not sure the Haynes manual is correct.

Clifton Gordon

Bruce, Clifton, Les, Pete....

Thanks for the tips on bleeding the brakes. I'll "read-up" on the how-to's in the Haynes manual first. And the old archieves.

However, as long as I'm doing this I might as well change the hoses too... and purchase the copper washer parts. And fill with Castrol LMA fluid.

Another question?? If I am only using my MGB as an occasional driver, not competition, is the cost of the steel braided hoses really worth it??

Donald Wheeler

Don. Remember the old saw about opinions and rectums, everyone has one. The choice of the straight rubber vs braided stainless sheathed rubber falls into the category of opinion and there are quite good reasons for having either opinion.

As Pete mentions, people using the covered (armoured) brake hoses point out that rubber swells under any conditions and the steel mesh covering provides a firmer pedal by limiting the ability of the rubber hose to expand. They can also point out that the steel mesh protects the rubber hose from damage due to rocks and debris on the road. Finally, if two rubber hoses are in marginal condition and about to break, the mess covered hose will prevent the rubber from swelling to the point of a catastropic failure, limiting the immediate damage to a leak where the plain hose would totally fail giving a complete loss of the front or rear braking system.

Those of us who use the rubber only lines point out that you can inspect the plain rubber lines to see what condition you they are in--the mesh allows cracking to take place out of sight. The rubber lines cost significantly less than the braid covered ones, hence, people are inclined to change them more often. Once a leak developes, it is immediately obvious where it is coming from with a rubber hose. Minor leaks may not show up well with a braided covering. Finally, yes the braking system feels a bit "tighter" with the braid covered lines. However, the average individual asked to test drive four cars, two with rubber flex lines and two with braid covered rubber flex lines would have some difficulty deciding which cars had the steel covered lines--they are only a single component in a complex system and rear brake ajustment, condition of the calipers/wheel cylinders, glazing of the pads/shoes are all as important as the type of flex line.

You should be able to make some degree of an informed decision based on the above. Here in Arizona, rubber lines deteriorate faster than in places with more moisture and less heat. Hence, my cars have the standard lines and are carefully inspected every three months during the oil change/lubrication/general inspection.

Les Bengtson


Thanks for the reply! I think I'll put new hoses on (standard rubber hoses). Your point about being able to spot if a leak is occuring is a good one.

Donald Wheeler

I had some rubbing coming from my brakes on the maiden voyages of my 67 GT, so today was a break job day.

I forgot to take pictures, but one of the rear brake cylinder pistons was stuck and all the pistons were rusty. I am sure this was due to the fact the car has not been running for a few years, but this is what the DOT 3 and DOT 4 brake fluids end up doing.

I flushed the system and replaced it all with DOT 5 silicon. Know this is one of those opinion things, but this one thing is the primary reason I have used silicon for 20 + years.

Also, a reminder. the outside piston on the disk brakes does not have a bleeder. You have to take it apart to make sure you get all the fluid out of it.

Not sure if a suction system will get this out.

Oh BTW. PO put one of the front disks in backwards. Yep, metal to metal. Oh well, I was going to put new rotors on it anyway.

Anyone know what the minimum thickness the shops will go to on a B Rotor?
Bruce Cunha

Repeat after me--B-R-A-K-E, BRAKE, Brake, brake.

Les Bengtson

B disks are of marginal thickness new, shops frequently don't cut them correctly. Don't mess with cutting them, new are very little more than the machine work costs anyway. The part that can be put in backasswards is a PAD, the thing that goes around is a disc, or rotor in yankeeland.
FR Millmore

Sorry FR and Les

I am really not illiterate, just type to fast and have become to dependent on spell check. Absolutly. Pad, Roter, Caliper.

Bruce Cunha

For tools, I made a 'one-man-brake-bleeder" out of a sweet pickle bottle. Guess a dill bottle would work too as long as it has a metal top. Drilled 3 holes on the top. One for 1/4" steel brake line and two for 5/16" Soldered one piece of 5/16" in one hole, sticking about 1/4" below the top and 1 1/2" or so above. Soldered one 1/4" and one 5/16" in the other holes so they were about 1/2" from the bottom of the jar and 1 1/2" or so above the metal top. Put an inch or so of water in the bottle and connect a rubber hose from the bleeder screw to the right size LONG steel line, open the bleeder screw and slowly pump the brake petal full strokes up and down. Will force fluid through the lines and will not suck air or water back. Make SURE you keep the master cyl full. Tighten the bleeder screw when there's no more air bubbles & you're done.
Another use for this is vacuum leak testing. If you put the vacuum source hose to the short line and the hose to the unit being tested on one of the other lines you can see when air is flowing (water bubbles up). Works great for leaking vacuum advances and brake boosters.

Bruce. Know exactly what you mean. Have been having the same problem for years now. When the brain is following a trend of thought and the fingers are trying to record what the brain is saying, they do not take dicatation as well as I should like.

Les Bengtson

Ya Yeah ya,.
My tongue got in the way of my eye teeth, and I couldn't see what I was sayin'

Safety Fast

Cliff, thanks for the tip. I had always wondered why my rear brakes were such a pig to bleed. Spent hours pumping away for no good effect. I guess now that the hydrolic part of the switch was moving back and forth and negating most of my efforts.
I actually ended up getting a large 50ml syringe, filling it with brake fluid and via a bit of rubber tubing injecting into the rear brake line at the master. This actually works very well, it's a pity that manufacturers don't put a T piece in for that purpose. I disconnected and plugged the master while injecting.

This thread was discussed between 17/09/2005 and 21/09/2005

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