Alfa Romeo Forums banner

1 - 11 of 11 Posts

·
Registered
Joined
·
684 Posts
Discussion Starter #1
I am having a hard time with the original brake booster that was factory installed on the 2600. I fully restored the unit. I had the piston housing re sleeved and installed a new kit supplied by Mike in florida. When the car is stationary the unit works well( very powerful boost) , but when after I drive the car for a while and use the brakes several time, and especially after hard braking, the booster locks up and does not release . A couple times I have been able to free it, but most of the time i end up disconnecting it to be able to keep on driving. The metal bell where the vacuum takes place is in excellent condition and i even took it apart to put a bit of vaseline to help the large piston travel. I would appreciate all comments regarding this...I have the felling that i have missed something on the rebuilt..help would be greatly appreciated as i have to rebuilt a second one for my other spider
 

·
Registered
Joined
·
9,641 Posts
Any chance you may have a spring installed upside down as described here? Keep in mind that there were changes in the booster from MKII to MKIIa that had to do with the "T" control valve, possibly because of issues like the ones mentioned here. FWIW, the Sunbeam Tiger uses the same brake servo as the 2600.
 

·
Registered
Joined
·
684 Posts
Discussion Starter #3
Thanks for the link Ruedi. When a control valve is mentioned..is it the one attached to the vaccum tank? I understand about the T valve..but where is that control valve? Like the case mentioned in the link, once I disconnect the air hose the large piston comes back.
 

·
Registered
Joined
·
9,641 Posts
This page describes how the MkII vacuum servo functions. The "T" control valve consists of a control piston and a "T" lever. It is important to understand that the control piston moves the "T" lever according to the pressure differential in the hydraulic lines and that the "T" lever controls 2 air valves (one being the air inlet for atmospheric pressure, the other for vacuum from the intake manifold).

If your servo gets stuck, the issue may relate to either the master piston being stuck (high friction in the vacuum container or weak spring) or the control piston/"T"-lever mechanism not allowing the pressure on both sides of the vacuum piston to equalize when you remove pedal pressure. Removal of the vacuum hose may lead to relief in both cases because the springs won't have to overcome friction AND any residual pressure differentials.

I vaguely recall I read somewhere the changes from MkII to MkIIa and then MkIIb servos had to do with modifications of the control piston/"T"-lever mechanism for smoother engagement of the brake assist (early systems seem to have engaged rather abruptly) but it's also possible that these modifications helped with issues like the one you're dealing with.

One way or the other, from what I read (I have no direct experience in dealing with them other than giving mine to a shop for overhauling) these servos seem to be rather finicky.
 

·
Registered
Joined
·
684 Posts
Discussion Starter #6
Hi Ruedi
Once more you have been a great help.. I found the issue of practical classics magazine..I will be studying it very carefully and most likely pull the booster out.. I probably will have to take my intake plenum and possibly the front carb to get it out..
Thanks a lot Ruedi
regards
Herve
 

·
Premium Member
Joined
·
435 Posts
Hi Herve
I posted the same fault on 20th june,it is almost word for word the same as yours,and I would be very interested in your findings as I had no replies and dont know what to do.If you resolve this problem please post the answer.
Chris
 

·
Registered
Joined
·
300 Posts
Herve and Chris, refer to Ruedi’s post above (post #5) and click on the link “this page” and read it carefully, especially the paragraphs under the diagram because I think it shows the solution to the problem you are having. It reads, “sometimes the piston can jam…” etc. They are referring to the large piston in the vacuum chamber, which is pushed back by the huge coil spring. In 2003, when finishing the brake system on my 2600 Spider, I had the same problem and, after removing the booster several times, I made a test fixture to test it on the bench. At the time, I was living in a metropolitan area of south Florida where it was very difficult to test drive my project and nearly impossible to trouble-shoot the brake system, which locked up at nearly each stop light! I was reduced to getting up at 2:00 AM and taking the car out on the roads until about 4:30 and trying to get home before the rush started at dawn. Anyway, that was my justification for going to the trouble to build this test rig. I used a vacuum pump and vacuum gauge and a high-pressure gauge reading to 4000 psi and a spare master cylinder with a long rod to simulate foot-pedal pressure. What I found is the same as others have found in the last many years and that is that the rebuild kits use a sponge rubber strip that is too large in diameter to expand the leather seal out against the vacuum chamber and it fits so tight that the huge coil spring can not return the piston when you let off the brake pedal. My solution was to slice off part of the sponge rubber expander strip with a razor, until I found the size that allowed the spring to return the piston and, at the same time, pressed hard enough that the vacuum would still allow the piston to work. I will try to post pictures of the test rig. BTW, I now live in rural north Florida, where there are nearly deserted country roads, perfect for us car nuts to test drive a car. This morning, I drove the Alfa about 20 miles, during which I twice tested the brakes from 60 mph. by braking at the threshold of lock-up; no other cars in sight!
Larry
 

Attachments

·
Premium Member
Joined
·
435 Posts
Hi Larry
Thanks for the information,all has become clear now and I have renewed interest in getting this sorted,I will post results when I get back from holiday.
Thanks again Chris
 

·
Registered
Joined
·
9,641 Posts
Here are some pages from a Girling service manual describing how the Girling Mk II brake booster operates and how (and how often) it should be serviced:
 

Attachments

1 - 11 of 11 Posts
Top